7 Expressions [expr]

7.1 Preamble [expr.pre]

1
#
[Note 1:
[expr] defines the syntax, order of evaluation, and meaning of expressions.52
An expression is a sequence of operators and operands that specifies a computation.
An expression can result in a value and can cause side effects.
— end note]
2
#
[Note 2:
Operators can be overloaded, that is, given meaning when applied to expressions of class type or enumeration type.
Uses of overloaded operators are transformed into function calls as described in [over.oper].
Overloaded operators obey the rules for syntax and evaluation order specified in [expr.compound], but the requirements of operand type and value category are replaced by the rules for function call.
Relations between operators, such as ++a meaning a+=1, are not guaranteed for overloaded operators.
— end note]
3
#
Subclause [expr.compound] defines the effects of operators when applied to types for which they have not been overloaded.
Operator overloading shall not modify the rules for the built-in operators, that is, for operators applied to types for which they are defined by this Standard.
However, these built-in operators participate in overload resolution, and as part of that process user-defined conversions will be considered where necessary to convert the operands to types appropriate for the built-in operator.
If a built-in operator is selected, such conversions will be applied to the operands before the operation is considered further according to the rules in subclause [expr.compound]; see [over.match.oper], [over.built].
4
#
If during the evaluation of an expression, the result is not mathematically defined or not in the range of representable values for its type, the behavior is undefined.
[Note 3:
Treatment of division by zero, forming a remainder using a zero divisor, and all floating-point exceptions varies among machines, and is sometimes adjustable by a library function.
— end note]
5
#
[Note 4:
The implementation can regroup operators according to the usual mathematical rules only where the operators really are associative or commutative.53
For example, in the following fragment int a, b; /* ... */ a = a + 32760 + b + 5; the expression statement behaves exactly the same as a = (((a + 32760) + b) + 5); due to the associativity and precedence of these operators.
Thus, the result of the sum (a + 32760) is next added to b, and that result is then added to 5 which results in the value assigned to a.
On a machine in which overflows produce an exception and in which the range of values representable by an int is [-32768, +32767], the implementation cannot rewrite this expression as a = ((a + b) + 32765); since if the values for a and b were, respectively, -32754 and -15, the sum a + b would produce an exception while the original expression would not; nor can the expression be rewritten as either a = ((a + 32765) + b); or a = (a + (b + 32765)); since the values for a and b might have been, respectively, 4 and -8 or -17 and 12.
However on a machine in which overflows do not produce an exception and in which the results of overflows are reversible, the above expression statement can be rewritten by the implementation in any of the above ways because the same result will occur.
— end note]
6
#
The values of the floating-point operands and the results of floating-point expressions may be represented in greater precision and range than that required by the type; the types are not changed thereby.54
52)
The precedence of operators is not directly specified, but it can be derived from the syntax.
 ⮥
53)
Overloaded operators are never assumed to be associative or commutative.
 ⮥
54)
The cast and assignment operators must still perform their specific conversions as described in [expr.type.conv], [expr.cast], [expr.static.cast] and [expr.ass].
 ⮥

7.2 Properties of expressions [expr.prop]

7.2.1 Value category [basic.lval]

1
#
Expressions are categorized according to the taxonomy in Figure 1.
categories expression expression glvalue glvalue expression->glvalue rvalue rvalue expression->rvalue lvalue lvalue glvalue->lvalue xvalue xvalue glvalue->xvalue rvalue->xvalue prvalue prvalue rvalue->prvalue
Figure 1: Expression category taxonomy  [fig:basic.lval]
  • (1.1)
    A glvalue is an expression whose evaluation determines the identity of an object or function.
  • (1.2)
    A prvalue is an expression whose evaluation initializes an object or computes the value of an operand of an operator, as specified by the context in which it appears, or an expression that has type cv void.
  • (1.3)
    An xvalue is a glvalue that denotes an object whose resources can be reused (usually because it is near the end of its lifetime).
  • (1.4)
    An lvalue is a glvalue that is not an xvalue.
  • (1.5)
    An rvalue is a prvalue or an xvalue.
2
#
Every expression belongs to exactly one of the fundamental classifications in this taxonomy: lvalue, xvalue, or prvalue.
This property of an expression is called its value category.
[Note 1:
The discussion of each built-in operator in [expr.compound] indicates the category of the value it yields and the value categories of the operands it expects.
For example, the built-in assignment operators expect that the left operand is an lvalue and that the right operand is a prvalue and yield an lvalue as the result.
User-defined operators are functions, and the categories of values they expect and yield are determined by their parameter and return types.
— end note]
3
#
[Note 2:
Historically, lvalues and rvalues were so-called because they could appear on the left- and right-hand side of an assignment (although this is no longer generally true); glvalues are “generalized” lvalues, prvalues are “pure” rvalues, and xvalues are “eXpiring” lvalues.
Despite their names, these terms classify expressions, not values.
— end note]
4
#
[Note 3:
An expression is an xvalue if it is:
In general, the effect of this rule is that named rvalue references are treated as lvalues and unnamed rvalue references to objects are treated as xvalues; rvalue references to functions are treated as lvalues whether named or not.
— end note]
[Example 1: struct A { int m; }; A&& operator+(A, A); A&& f(); A a; A&& ar = static_cast<A&&>(a);
The expressions f(), f().m, static_­cast<A&&>(a), and a + a are xvalues.
The expression ar is an lvalue.
— end example]
5
#
The result of a glvalue is the entity denoted by the expression.
The result of a prvalue is the value that the expression stores into its context; a prvalue that has type cv void has no result.
A prvalue whose result is the value V is sometimes said to have or name the value V.
The result object of a prvalue is the object initialized by the prvalue; a non-discarded prvalue that is used to compute the value of an operand of a built-in operator or a prvalue that has type cv void has no result object.
[Note 4:
Except when the prvalue is the operand of a decltype-specifier, a prvalue of class or array type always has a result object.
For a discarded prvalue that has type other than cv void, a temporary object is materialized; see [expr.context].
— end note]
6
#
Whenever a glvalue appears as an operand of an operator that expects a prvalue for that operand, the lvalue-to-rvalue, array-to-pointer, or function-to-pointer standard conversions are applied to convert the expression to a prvalue.
[Note 5:
An attempt to bind an rvalue reference to an lvalue is not such a context; see [dcl.init.ref].
— end note]
[Note 6:
Because cv-qualifiers are removed from the type of an expression of non-class type when the expression is converted to a prvalue, an lvalue of type const int can, for example, be used where a prvalue of type int is required.
— end note]
[Note 7:
There are no prvalue bit-fields; if a bit-field is converted to a prvalue ([conv.lval]), a prvalue of the type of the bit-field is created, which might then be promoted ([conv.prom]).
— end note]
7
#
Whenever a prvalue appears as an operand of an operator that expects a glvalue for that operand, the temporary materialization conversion is applied to convert the expression to an xvalue.
8
#
The discussion of reference initialization in [dcl.init.ref] and of temporaries in [class.temporary] indicates the behavior of lvalues and rvalues in other significant contexts.
9
#
Unless otherwise indicated ([dcl.type.decltype]), a prvalue shall always have complete type or the void type; if it has a class type or (possibly multi-dimensional) array of class type, that class shall not be an abstract class ([class.abstract]).
A glvalue shall not have type cv void.
[Note 8:
A glvalue can have complete or incomplete non-void type.
Class and array prvalues can have cv-qualified types; other prvalues always have cv-unqualified types.
— end note]
10
#
An lvalue is modifiable unless its type is const-qualified or is a function type.
[Note 9:
A program that attempts to modify an object through a nonmodifiable lvalue or through an rvalue is ill-formed ([expr.ass], [expr.post.incr], [expr.pre.incr]).
— end note]
11
#
If a program attempts to access the stored value of an object through a glvalue whose type is not similar to one of the following types the behavior is undefined:55
  • (11.1)
    the dynamic type of the object,
  • (11.2)
    a type that is the signed or unsigned type corresponding to the dynamic type of the object, or
  • (11.3)
    a char, unsigned char, or std​::​byte type.
If a program invokes a defaulted copy/move constructor or copy/move assignment operator for a union of type U with a glvalue argument that does not denote an object of type cv U within its lifetime, the behavior is undefined.
[Note 10:
Unlike in C, C++ has no accesses of class type.
— end note]
55)
The intent of this list is to specify those circumstances in which an object can or cannot be aliased.
 ⮥

7.2.2 Type [expr.type]

1
#
If an expression initially has the type “reference to T” ([dcl.ref], [dcl.init.ref]), the type is adjusted to T prior to any further analysis.
The expression designates the object or function denoted by the reference, and the expression is an lvalue or an xvalue, depending on the expression.
[Note 1:
Before the lifetime of the reference has started or after it has ended, the behavior is undefined (see [basic.life]).
— end note]
2
#
If a prvalue initially has the type “cv T”, where T is a cv-unqualified non-class, non-array type, the type of the expression is adjusted to T prior to any further analysis.
3
#
The composite pointer type of two operands p1 and p2 having types T1 and T2, respectively, where at least one is a pointer or pointer-to-member type or std​::​nullptr_­t, is:
  • (3.1)
    if both p1 and p2 are null pointer constants, std​::​nullptr_­t;
  • (3.2)
    if either p1 or p2 is a null pointer constant, T2 or T1, respectively;
  • (3.3)
    if T1 or T2 is “pointer to cv1 void” and the other type is “pointer to cv2 T”, where T is an object type or void, “pointer to cv12 void”, where cv12 is the union of cv1 and cv2;
  • (3.4)
    if T1 or T2 is “pointer to noexcept function” and the other type is “pointer to function”, where the function types are otherwise the same, “pointer to function”;
  • (3.5)
    if T1 is “pointer to cv1 C1” and T2 is “pointer to cv2 C2”, where C1 is reference-related to C2 or C2 is reference-related to C1, the cv-combined type of T1 and T2 or the cv-combined type of T2 and T1, respectively;
  • (3.6)
    if T1 or T2 is “pointer to member of C1 of type function”, the other type is “pointer to member of C2 of type noexcept function”, and C1 is reference-related to C2 or C2 is reference-related to C1, where the function types are otherwise the same, “pointer to member of C2 of type function” or “pointer to member of C1 of type function”, respectively;
  • (3.7)
    if T1 is “pointer to member of C1 of type cv1 U” and T2 is “pointer to member of C2 of type cv2 U”, for some non-function type U, where C1 is reference-related to C2 or C2 is reference-related to C1, the cv-combined type of T2 and T1 or the cv-combined type of T1 and T2, respectively;
  • (3.8)
    if T1 and T2 are similar types, the cv-combined type of T1 and T2;
  • (3.9)
    otherwise, a program that necessitates the determination of a composite pointer type is ill-formed.
[Example 1: typedef void *p; typedef const int *q; typedef int **pi; typedef const int **pci;
The composite pointer type of p and q is “pointer to const void”; the composite pointer type of pi and pci is “pointer to const pointer to const int”.
— end example]

7.2.3 Context dependence [expr.context]

1
#
An unevaluated operand is not evaluated.
[Note 1:
In an unevaluated operand, a non-static class member can be named ([expr.prim.id]) and naming of objects or functions does not, by itself, require that a definition be provided ([basic.def.odr]).
An unevaluated operand is considered a full-expression.
— end note]
2
#
In some contexts, an expression only appears for its side effects.
Such an expression is called a discarded-value expression.
The array-to-pointer and function-to-pointer standard conversions are not applied.
The lvalue-to-rvalue conversion is applied if and only if the expression is a glvalue of volatile-qualified type and it is one of the following:
[Note 2:
Using an overloaded operator causes a function call; the above covers only operators with built-in meaning.
— end note]
If the (possibly converted) expression is a prvalue, the temporary materialization conversion is applied.
[Note 3:
If the expression is an lvalue of class type, it must have a volatile copy constructor to initialize the temporary object that is the result object of the lvalue-to-rvalue conversion.
— end note]
The glvalue expression is evaluated and its value is discarded.

7.3 Standard conversions [conv]

7.3.1 General [conv.general]

1
#
Standard conversions are implicit conversions with built-in meaning.
[conv] enumerates the full set of such conversions.
A standard conversion sequence is a sequence of standard conversions in the following order:
  • (1.1)
    Zero or one conversion from the following set: lvalue-to-rvalue conversion, array-to-pointer conversion, and function-to-pointer conversion.
  • (1.2)
    Zero or one conversion from the following set: integral promotions, floating-point promotion, integral conversions, floating-point conversions, floating-integral conversions, pointer conversions, pointer-to-member conversions, and boolean conversions.
  • (1.3)
    Zero or one function pointer conversion.
  • (1.4)
    Zero or one qualification conversion.
[Note 1:
A standard conversion sequence can be empty, i.e., it can consist of no conversions.
— end note]
A standard conversion sequence will be applied to an expression if necessary to convert it to a required destination type.
2
#
[Note 2:
Expressions with a given type will be implicitly converted to other types in several contexts:
  • (2.1)
    When used as operands of operators.
    The operator's requirements for its operands dictate the destination type ([expr.compound]).
  • (2.2)
    When used in the condition of an if statement ([stmt.if]) or iteration statement ([stmt.iter]).
    The destination type is bool.
  • (2.3)
    When used in the expression of a switch statement ([stmt.switch]).
    The destination type is integral.
  • (2.4)
    When used as the source expression for an initialization (which includes use as an argument in a function call and use as the expression in a return statement).
    The type of the entity being initialized is (generally) the destination type.
— end note]
3
#
An expression E can be implicitly converted to a type T if and only if the declaration T t=E; is well-formed, for some invented temporary variable t ([dcl.init]).
4
#
Certain language constructs require that an expression be converted to a Boolean value.
An expression E appearing in such a context is said to be contextually converted to bool and is well-formed if and only if the declaration bool t(E); is well-formed, for some invented temporary variable t ([dcl.init]).
5
#
Certain language constructs require conversion to a value having one of a specified set of types appropriate to the construct.
An expression E of class type C appearing in such a context is said to be contextually implicitly converted to a specified type T and is well-formed if and only if E can be implicitly converted to a type T that is determined as follows: C is searched for non-explicit conversion functions whose return type is cv T or reference to cv T such that T is allowed by the context.
There shall be exactly one such T.
6
#
The effect of any implicit conversion is the same as performing the corresponding declaration and initialization and then using the temporary variable as the result of the conversion.
The result is an lvalue if T is an lvalue reference type or an rvalue reference to function type ([dcl.ref]), an xvalue if T is an rvalue reference to object type, and a prvalue otherwise.
The expression E is used as a glvalue if and only if the initialization uses it as a glvalue.
7
#
[Note 3:
For class types, user-defined conversions are considered as well; see [class.conv].
In general, an implicit conversion sequence ([over.best.ics]) consists of a standard conversion sequence followed by a user-defined conversion followed by another standard conversion sequence.
— end note]
8
#
[Note 4:
There are some contexts where certain conversions are suppressed.
For example, the lvalue-to-rvalue conversion is not done on the operand of the unary & operator.
Specific exceptions are given in the descriptions of those operators and contexts.
— end note]

7.3.2 Lvalue-to-rvalue conversion [conv.lval]

1
#
A glvalue of a non-function, non-array type T can be converted to a prvalue.56
If T is an incomplete type, a program that necessitates this conversion is ill-formed.
If T is a non-class type, the type of the prvalue is the cv-unqualified version of T.
Otherwise, the type of the prvalue is T.57
2
#
When an lvalue-to-rvalue conversion is applied to an expression E, and either
  • (2.1)
    E is not potentially evaluated, or
  • (2.2)
    the evaluation of E results in the evaluation of a member of the set of potential results of E, and names a variable x that is not odr-used by ([basic.def.odr]),
the value contained in the referenced object is not accessed.
[Example 1: struct S { int n; }; auto f() { S x { 1 }; constexpr S y { 2 }; return [&](bool b) { return (b ? y : x).n; }; } auto g = f(); int m = g(false); // undefined behavior: access of x.n outside its lifetime int n = g(true); // OK, does not access y.n — end example]
3
#
The result of the conversion is determined according to the following rules:
  • (3.1)
    If T is cv std​::​nullptr_­t, the result is a null pointer constant ([conv.ptr]).
    [Note 1:
    Since the conversion does not access the object to which the glvalue refers, there is no side effect even if T is volatile-qualified ([intro.execution]), and the glvalue can refer to an inactive member of a union ([class.union]).
    — end note]
  • (3.2)
    Otherwise, if T has a class type, the conversion copy-initializes the result object from the glvalue.
  • (3.3)
    Otherwise, if the object to which the glvalue refers contains an invalid pointer value ([basic.stc.dynamic.deallocation], [basic.stc.dynamic.safety]), the behavior is implementation-defined.
  • (3.4)
    Otherwise, the object indicated by the glvalue is read ([defns.access]), and the value contained in the object is the prvalue result.
4
#
[Note 2:
See also [basic.lval].
— end note]
56)
For historical reasons, this conversion is called the “lvalue-to-rvalue” conversion, even though that name does not accurately reflect the taxonomy of expressions described in [basic.lval].
 ⮥
57)
In C++ class and array prvalues can have cv-qualified types.
This differs from ISO C, in which non-lvalues never have cv-qualified types.
 ⮥

7.3.3 Array-to-pointer conversion [conv.array]

1
#
An lvalue or rvalue of type “array of N T” or “array of unknown bound of T” can be converted to a prvalue of type “pointer to T”.
The temporary materialization conversion ([conv.rval]) is applied.
The result is a pointer to the first element of the array.

7.3.4 Function-to-pointer conversion [conv.func]

1
#
An lvalue of function type T can be converted to a prvalue of type “pointer to T”.
The result is a pointer to the function.58
58)
This conversion never applies to non-static member functions because an lvalue that refers to a non-static member function cannot be obtained.
 ⮥

7.3.5 Temporary materialization conversion [conv.rval]

1
#
A prvalue of type T can be converted to an xvalue of type T.
This conversion initializes a temporary object ([class.temporary]) of type T from the prvalue by evaluating the prvalue with the temporary object as its result object, and produces an xvalue denoting the temporary object.
T shall be a complete type.
[Note 1:
If T is a class type (or array thereof), it must have an accessible and non-deleted destructor; see [class.dtor].
— end note]
[Example 1: struct X { int n; }; int k = X().n; // OK, X() prvalue is converted to xvalue — end example]

7.3.6 Qualification conversions [conv.qual]

1
#
A cv-decomposition of a type T is a sequence of and such that T is “ ⋯ U” for , where each is a set of cv-qualifiers ([basic.type.qualifier]), and each is “pointer to” ([dcl.ptr]), “pointer to member of class of type” ([dcl.mptr]), “array of ”, or “array of unknown bound of” ([dcl.array]).
If designates an array, the cv-qualifiers on the element type are also taken as the cv-qualifiers of the array.
[Example 1:
The type denoted by the type-id const int ** has three cv-decompositions, taking U as “int”, as “pointer to const int”, and as “pointer to pointer to const int”.
— end example]
The n-tuple of cv-qualifiers after the first one in the longest cv-decomposition of T, that is, , is called the cv-qualification signature of T.
2
#
Two types T1 and T2 are similar if they have cv-decompositions with the same n such that corresponding components are either the same or one is “array of ” and the other is “array of unknown bound of”, and the types denoted by U are the same.
3
#
The cv-combined type of two types T1 and T2 is the type T3 similar to T1 whose cv-decomposition is such that:
  • (3.1)
    for every , is the union of and ,
  • (3.2)
    if either or is “array of unknown bound of”, is “array of unknown bound of”, otherwise it is , and
  • (3.3)
    if the resulting is different from or , or the resulting is different from or , then const is added to every for ,
where and are the components of the cv-decomposition of Tj.
A prvalue of type T1 can be converted to type T2 if the cv-combined type of T1 and T2 is T2.
[Note 1:
If a program could assign a pointer of type T** to a pointer of type const T** (that is, if line #1 below were allowed), a program could inadvertently modify a const object (as it is done on line #2).
For example, int main() { const char c = 'c'; char* pc; const char** pcc = &pc; // #1: not allowed *pcc = &c; *pc = 'C'; // #2: modifies a const object }
— end note]
[Note 2:
Given similar types T1 and T2, this construction ensures that both can be converted to the cv-combined type of T1 and T2.
— end note]
4
#
[Note 3:
A prvalue of type “pointer to cv1 T” can be converted to a prvalue of type “pointer to cv2 T” if “cv2 T” is more cv-qualified than “cv1 T”.
A prvalue of type “pointer to member of X of type cv1 T” can be converted to a prvalue of type “pointer to member of X of type cv2 T” if “cv2 T” is more cv-qualified than “cv1 T”.
— end note]
5
#
[Note 4:
Function types (including those used in pointer-to-member-function types) are never cv-qualified ([dcl.fct]).
— end note]

7.3.7 Integral promotions [conv.prom]

1
#
A prvalue of an integer type other than bool, char16_­t, char32_­t, or wchar_­t whose integer conversion rank ([conv.rank]) is less than the rank of int can be converted to a prvalue of type int if int can represent all the values of the source type; otherwise, the source prvalue can be converted to a prvalue of type unsigned int.
2
#
A prvalue of type char16_­t, char32_­t, or wchar_­t ([basic.fundamental]) can be converted to a prvalue of the first of the following types that can represent all the values of its underlying type: int, unsigned int, long int, unsigned long int, long long int, or unsigned long long int.
If none of the types in that list can represent all the values of its underlying type, a prvalue of type char16_­t, char32_­t, or wchar_­t can be converted to a prvalue of its underlying type.
3
#
A prvalue of an unscoped enumeration type whose underlying type is not fixed can be converted to a prvalue of the first of the following types that can represent all the values of the enumeration ([dcl.enum]): int, unsigned int, long int, unsigned long int, long long int, or unsigned long long int.
If none of the types in that list can represent all the values of the enumeration, a prvalue of an unscoped enumeration type can be converted to a prvalue of the extended integer type with lowest integer conversion rank ([conv.rank]) greater than the rank of long long in which all the values of the enumeration can be represented.
If there are two such extended types, the signed one is chosen.
4
#
A prvalue of an unscoped enumeration type whose underlying type is fixed ([dcl.enum]) can be converted to a prvalue of its underlying type.
Moreover, if integral promotion can be applied to its underlying type, a prvalue of an unscoped enumeration type whose underlying type is fixed can also be converted to a prvalue of the promoted underlying type.
5
#
A prvalue for an integral bit-field ([class.bit]) can be converted to a prvalue of type int if int can represent all the values of the bit-field; otherwise, it can be converted to unsigned int if unsigned int can represent all the values of the bit-field.
If the bit-field is larger yet, no integral promotion applies to it.
If the bit-field has an enumerated type, it is treated as any other value of that type for promotion purposes.
6
#
A prvalue of type bool can be converted to a prvalue of type int, with false becoming zero and true becoming one.
7
#
These conversions are called integral promotions.

7.3.8 Floating-point promotion [conv.fpprom]

1
#
A prvalue of type float can be converted to a prvalue of type double.
The value is unchanged.
2
#
This conversion is called floating-point promotion.

7.3.9 Integral conversions [conv.integral]

1
#
A prvalue of an integer type can be converted to a prvalue of another integer type.
A prvalue of an unscoped enumeration type can be converted to a prvalue of an integer type.
2
#
If the destination type is bool, see [conv.bool].
If the source type is bool, the value false is converted to zero and the value true is converted to one.
3
#
Otherwise, the result is the unique value of the destination type that is congruent to the source integer modulo , where N is the width of the destination type.
4
#
The conversions allowed as integral promotions are excluded from the set of integral conversions.

7.3.10 Floating-point conversions [conv.double]

1
#
A prvalue of floating-point type can be converted to a prvalue of another floating-point type.
If the source value can be exactly represented in the destination type, the result of the conversion is that exact representation.
If the source value is between two adjacent destination values, the result of the conversion is an implementation-defined choice of either of those values.
Otherwise, the behavior is undefined.
2
#
The conversions allowed as floating-point promotions are excluded from the set of floating-point conversions.

7.3.11 Floating-integral conversions [conv.fpint]

1
#
A prvalue of a floating-point type can be converted to a prvalue of an integer type.
The conversion truncates; that is, the fractional part is discarded.
The behavior is undefined if the truncated value cannot be represented in the destination type.
[Note 1:
If the destination type is bool, see [conv.bool].
— end note]
2
#
A prvalue of an integer type or of an unscoped enumeration type can be converted to a prvalue of a floating-point type.
The result is exact if possible.
If the value being converted is in the range of values that can be represented but the value cannot be represented exactly, it is an implementation-defined choice of either the next lower or higher representable value.
[Note 2:
Loss of precision occurs if the integral value cannot be represented exactly as a value of the floating-point type.
— end note]
If the value being converted is outside the range of values that can be represented, the behavior is undefined.
If the source type is bool, the value false is converted to zero and the value true is converted to one.

7.3.12 Pointer conversions [conv.ptr]

1
#
A null pointer constant is an integer literal ([lex.icon]) with value zero or a prvalue of type std​::​nullptr_­t.
A null pointer constant can be converted to a pointer type; the result is the null pointer value of that type ([basic.compound]) and is distinguishable from every other value of object pointer or function pointer type.
Such a conversion is called a null pointer conversion.
Two null pointer values of the same type shall compare equal.
The conversion of a null pointer constant to a pointer to cv-qualified type is a single conversion, and not the sequence of a pointer conversion followed by a qualification conversion ([conv.qual]).
A null pointer constant of integral type can be converted to a prvalue of type std​::​nullptr_­t.
[Note 1:
The resulting prvalue is not a null pointer value.
— end note]
2
#
A prvalue of type “pointer to cv T”, where T is an object type, can be converted to a prvalue of type “pointer to cv void”.
The pointer value ([basic.compound]) is unchanged by this conversion.
3
#
A prvalue of type “pointer to cv D”, where D is a complete class type, can be converted to a prvalue of type “pointer to cv B”, where B is a base class ([class.derived]) of D.
If B is an inaccessible ([class.access]) or ambiguous ([class.member.lookup]) base class of D, a program that necessitates this conversion is ill-formed.
The result of the conversion is a pointer to the base class subobject of the derived class object.
The null pointer value is converted to the null pointer value of the destination type.

7.3.13 Pointer-to-member conversions [conv.mem]

1
#
A null pointer constant can be converted to a pointer-to-member type; the result is the null member pointer value of that type and is distinguishable from any pointer to member not created from a null pointer constant.
Such a conversion is called a null member pointer conversion.
Two null member pointer values of the same type shall compare equal.
The conversion of a null pointer constant to a pointer to member of cv-qualified type is a single conversion, and not the sequence of a pointer-to-member conversion followed by a qualification conversion ([conv.qual]).
2
#
A prvalue of type “pointer to member of B of type cv T”, where B is a class type, can be converted to a prvalue of type “pointer to member of D of type cv T”, where D is a complete class derived ([class.derived]) from B.
If B is an inaccessible ([class.access]), ambiguous ([class.member.lookup]), or virtual ([class.mi]) base class of D, or a base class of a virtual base class of D, a program that necessitates this conversion is ill-formed.
The result of the conversion refers to the same member as the pointer to member before the conversion took place, but it refers to the base class member as if it were a member of the derived class.
The result refers to the member in D's instance of B.
Since the result has type “pointer to member of D of type cv T”, indirection through it with a D object is valid.
The result is the same as if indirecting through the pointer to member of B with the B subobject of D.
The null member pointer value is converted to the null member pointer value of the destination type.59
59)
The rule for conversion of pointers to members (from pointer to member of base to pointer to member of derived) appears inverted compared to the rule for pointers to objects (from pointer to derived to pointer to base) ([conv.ptr], [class.derived]).
This inversion is necessary to ensure type safety.
Note that a pointer to member is not an object pointer or a function pointer and the rules for conversions of such pointers do not apply to pointers to members.
In particular, a pointer to member cannot be converted to a void*.
 ⮥

7.3.14 Function pointer conversions [conv.fctptr]

1
#
A prvalue of type “pointer to noexcept function” can be converted to a prvalue of type “pointer to function”.
The result is a pointer to the function.
A prvalue of type “pointer to member of type noexcept function” can be converted to a prvalue of type “pointer to member of type function”.
The result designates the member function.
[Example 1: void (*p)(); void (**pp)() noexcept = &p; // error: cannot convert to pointer to noexcept function struct S { typedef void (*p)(); operator p(); }; void (*q)() noexcept = S(); // error: cannot convert to pointer to noexcept function — end example]

7.3.15 Boolean conversions [conv.bool]

1
#
A prvalue of arithmetic, unscoped enumeration, pointer, or pointer-to-member type can be converted to a prvalue of type bool.
A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.

7.4 Usual arithmetic conversions [expr.arith.conv]

1
#
Many binary operators that expect operands of arithmetic or enumeration type cause conversions and yield result types in a similar way.
The purpose is to yield a common type, which is also the type of the result.
This pattern is called the usual arithmetic conversions, which are defined as follows:
  • (1.1)
    If either operand is of scoped enumeration type, no conversions are performed; if the other operand does not have the same type, the expression is ill-formed.
  • (1.2)
    If either operand is of type long double, the other shall be converted to long double.
  • (1.3)
    Otherwise, if either operand is double, the other shall be converted to double.
  • (1.4)
    Otherwise, if either operand is float, the other shall be converted to float.
  • (1.5)
    Otherwise, the integral promotions ([conv.prom]) shall be performed on both operands.60
    Then the following rules shall be applied to the promoted operands:
    • (1.5.1)
      If both operands have the same type, no further conversion is needed.
    • (1.5.2)
      Otherwise, if both operands have signed integer types or both have unsigned integer types, the operand with the type of lesser integer conversion rank shall be converted to the type of the operand with greater rank.
    • (1.5.3)
      Otherwise, if the operand that has unsigned integer type has rank greater than or equal to the rank of the type of the other operand, the operand with signed integer type shall be converted to the type of the operand with unsigned integer type.
    • (1.5.4)
      Otherwise, if the type of the operand with signed integer type can represent all of the values of the type of the operand with unsigned integer type, the operand with unsigned integer type shall be converted to the type of the operand with signed integer type.
    • (1.5.5)
      Otherwise, both operands shall be converted to the unsigned integer type corresponding to the type of the operand with signed integer type.
2
#
If one operand is of enumeration type and the other operand is of a different enumeration type or a floating-point type, this behavior is deprecated ([depr.arith.conv.enum]).
60)
As a consequence, operands of type bool, char8_­t, char16_­t, char32_­t, wchar_­t, or an enumerated type are converted to some integral type.
 ⮥

7.5 Primary expressions [expr.prim]

7.5.1 Literals [expr.prim.literal]

1
#
A literal is a primary expression.
The type of a literal is determined based on its form as specified in [lex.literal].
A string-literal is an lvalue, a user-defined-literal has the same value category as the corresponding operator call expression described in [lex.ext], and any other literal is a prvalue.

7.5.2 This [expr.prim.this]

1
#
The keyword this names a pointer to the object for which a non-static member function ([class.this]) is invoked or a non-static data member's initializer ([class.mem]) is evaluated.
2
#
If a declaration declares a member function or member function template of a class X, the expression this is a prvalue of type “pointer to cv-qualifier-seq X” between the optional cv-qualifier-seq and the end of the function-definition, member-declarator, or declarator.
It shall not appear before the optional cv-qualifier-seq and it shall not appear within the declaration of a static member function (although its type and value category are defined within a static member function as they are within a non-static member function).
[Note 1:
This is because declaration matching does not occur until the complete declarator is known.
— end note]
[Note 2:
In a trailing-return-type, the class being defined is not required to be complete for purposes of class member access.
Class members declared later are not visible.
[Example 1: struct A { char g(); template<class T> auto f(T t) -> decltype(t + g()) { return t + g(); } }; template auto A::f(int t) -> decltype(t + g()); — end example]
— end note]
3
#
Otherwise, if a member-declarator declares a non-static data member of a class X, the expression this is a prvalue of type “pointer to X” within the optional default member initializer.
It shall not appear elsewhere in the member-declarator.
4
#
The expression this shall not appear in any other context.
[Example 2: class Outer { int a[sizeof(*this)]; // error: not inside a member function unsigned int sz = sizeof(*this); // OK: in default member initializer void f() { int b[sizeof(*this)]; // OK struct Inner { int c[sizeof(*this)]; // error: not inside a member function of Inner }; } }; — end example]

7.5.3 Parentheses [expr.prim.paren]

1
#
A parenthesized expression (E) is a primary expression whose type, result, and value category are identical to those of E.
The parenthesized expression can be used in exactly the same contexts as those where E can be used, and with the same meaning, except as otherwise indicated.

7.5.4 Names [expr.prim.id]

7.5.4.1 General [expr.prim.id.general]

1
#
An id-expression is a restricted form of a primary-expression.
[Note 1: — end note]
2
#
An id-expression that denotes a non-static data member or non-static member function of a class can only be used:
  • (2.1)
    as part of a class member access in which the object expression refers to the member's class61 or a class derived from that class, or
  • (2.2)
    to form a pointer to member ([expr.unary.op]), or
  • (2.3)
    if that id-expression denotes a non-static data member and it appears in an unevaluated operand.
    [Example 1: struct S { int m; }; int i = sizeof(S::m); // OK int j = sizeof(S::m + 42); // OK — end example]
3
#
A potentially-evaluated id-expression that denotes an immediate function shall appear only
4
#
For an id-expression that denotes an overload set, overload resolution is performed to select a unique function ([over.match], [over.over]).
[Note 2:
A program cannot refer to a function with a trailing requires-clause whose constraint-expression is not satisfied, because such functions are never selected by overload resolution.
[Example 2: template<typename T> struct A { static void f(int) requires false; } void g() { A<int>::f(0); // error: cannot call f void (*p1)(int) = A<int>::f; // error: cannot take the address of f decltype(A<int>::f)* p2 = nullptr; // error: the type decltype(A<int>​::​f) is invalid }
In each case, the constraints of f are not satisfied.
In the declaration of p2, those constraints are required to be satisfied even though f is an unevaluated operand.
— end example]
— end note]
61)
This also applies when the object expression is an implicit (*this) ([class.mfct.non-static]).
 ⮥

7.5.4.2 Unqualified names [expr.prim.id.unqual]

1
#
An identifier is only an id-expression if it has been suitably declared ([dcl.dcl]) or if it appears as part of a declarator-id ([dcl.decl]).
An identifier that names a coroutine parameter refers to the copy of the parameter ([dcl.fct.def.coroutine]).
[Note 1:
A type-name or decltype-specifier prefixed by ~ denotes the destructor of the type so named; see [expr.prim.id.dtor].
Within the definition of a non-static member function, an identifier that names a non-static member is transformed to a class member access expression ([class.mfct.non-static]).
— end note]
2
#
The result is the entity denoted by the identifier.
If the entity is a local entity and naming it from outside of an unevaluated operand within the declarative region where the unqualified-id appears would result in some intervening lambda-expression capturing it by copy ([expr.prim.lambda.capture]), the type of the expression is the type of a class member access expression ([expr.ref]) naming the non-static data member that would be declared for such a capture in the closure object of the innermost such intervening lambda-expression.
[Note 2:
If that lambda-expression is not declared mutable, the type of such an identifier will typically be const qualified.
— end note]
Otherwise, the type of the expression is the type of the result.
[Note 3:
If the entity is a template parameter object for a template parameter of type T ([temp.param]), the type of the expression is const T.
— end note]
[Note 4:
The type will be adjusted as described in [expr.type] if it is cv-qualified or is a reference type.
— end note]
The expression is an lvalue if the entity is a function, variable, structured binding, data member, or template parameter object and a prvalue otherwise ([basic.lval]); it is a bit-field if the identifier designates a bit-field.
[Example 1: void f() { float x, &r = x; [=] { decltype(x) y1; // y1 has type float decltype((x)) y2 = y1; // y2 has type float const& because this lambda // is not mutable and x is an lvalue decltype(r) r1 = y1; // r1 has type float& decltype((r)) r2 = y2; // r2 has type float const& }; } — end example]

7.5.4.3 Qualified names [expr.prim.id.qual]

1
#
The type denoted by a decltype-specifier in a nested-name-specifier shall be a class or enumeration type.
2
#
A nested-name-specifier that denotes a class, optionally followed by the keyword template ([temp.names]), and then followed by the name of a member of either that class ([class.mem]) or one of its base classes, is a qualified-id; [class.qual] describes name lookup for class members that appear in qualified-ids.
The result is the member.
The type of the result is the type of the member.
The result is an lvalue if the member is a static member function or a data member and a prvalue otherwise.
[Note 1:
A class member can be referred to using a qualified-id at any point in its potential scope ([basic.scope.class]).
— end note]
Where type-name ​::​~ type-name is used, the two type-names shall refer to the same type (ignoring cv-qualifications); this notation denotes the destructor of the type so named ([expr.prim.id.dtor]).
The unqualified-id in a qualified-id shall not be of the form ~decltype-specifier.
3
#
The nested-name-specifier ​::​ names the global namespace.
A nested-name-specifier that names a namespace ([basic.namespace]), optionally followed by the keyword template ([temp.names]), and then followed by the name of a member of that namespace (or the name of a member of a namespace made visible by a using-directive), is a qualified-id; [namespace.qual] describes name lookup for namespace members that appear in qualified-ids.
The result is the member.
The type of the result is the type of the member.
The result is an lvalue if the member is a function, a variable, or a structured binding ([dcl.struct.bind]) and a prvalue otherwise.
4
#
A nested-name-specifier that denotes an enumeration, followed by the name of an enumerator of that enumeration, is a qualified-id that refers to the enumerator.
The result is the enumerator.
The type of the result is the type of the enumeration.
The result is a prvalue.
5
#
In a qualified-id, if the unqualified-id is a conversion-function-id, its conversion-type-id is first looked up in the class denoted by the nested-name-specifier of the qualified-id and the name, if found, is used.
Otherwise, it is looked up in the context in which the entire qualified-id occurs.
In each of these lookups, only names that denote types or templates whose specializations are types are considered.

7.5.4.4 Destruction [expr.prim.id.dtor]

1
#
An id-expression that denotes the destructor of a type T names the destructor of T if T is a class type ([class.dtor]), otherwise the id-expression is said to name a pseudo-destructor.
2
#
If the id-expression names a pseudo-destructor, T shall be a scalar type and the id-expression shall appear as the right operand of a class member access ([expr.ref]) that forms the postfix-expression of a function call ([expr.call]).
[Note 1:
Such a call ends the lifetime of the object ([expr.call], [basic.life]).
— end note]
3
#
[Example 1: struct C { }; void f() { C * pc = new C; using C2 = C; pc->C::~C2(); // OK, destroys *pc C().C::~C(); // undefined behavior: temporary of type C destroyed twice using T = int; 0 .T::~T(); // OK, no effect 0.T::~T(); // error: 0.T is a user-defined-floating-point-literal ([lex.ext]) } — end example]

7.5.5 Lambda expressions [expr.prim.lambda]

7.5.5.1 General [expr.prim.lambda.general]

1
#
A lambda-expression provides a concise way to create a simple function object.
[Example 1: #include <algorithm> #include <cmath> void abssort(float* x, unsigned N) { std::sort(x, x + N, [](float a, float b) { return std::abs(a) < std::abs(b); }); } — end example]
2
#
A lambda-expression is a prvalue whose result object is called the closure object.
[Note 1:
A closure object behaves like a function object.
— end note]
3
#
In the decl-specifier-seq of the lambda-declarator, each decl-specifier shall be one of mutable, constexpr, or consteval.
[Note 2:
The trailing requires-clause is described in [dcl.decl].
— end note]
4
#
If a lambda-expression does not include a lambda-declarator, it is as if the lambda-declarator were ().
The lambda return type is auto, which is replaced by the type specified by the trailing-return-type if provided and/or deduced from return statements as described in [dcl.spec.auto].
[Example 2: auto x1 = [](int i){ return i; }; // OK: return type is int auto x2 = []{ return { 1, 2 }; }; // error: deducing return type from braced-init-list int j; auto x3 = []()->auto&& { return j; }; // OK: return type is int& — end example]
5
#
A lambda is a generic lambda if the lambda-expression has any generic parameter type placeholders ([dcl.spec.auto]), or if the lambda has a template-parameter-list.
[Example 3: int i = [](int i, auto a) { return i; }(3, 4); // OK: a generic lambda int j = []<class T>(T t, int i) { return i; }(3, 4); // OK: a generic lambda — end example]

7.5.5.2 Closure types [expr.prim.lambda.closure]

1
#
The type of a lambda-expression (which is also the type of the closure object) is a unique, unnamed non-union class type, called the closure type, whose properties are described below.
2
#
The closure type is declared in the smallest block scope, class scope, or namespace scope that contains the corresponding lambda-expression.
[Note 1:
This determines the set of namespaces and classes associated with the closure type ([basic.lookup.argdep]).
The parameter types of a lambda-declarator do not affect these associated namespaces and classes.
— end note]
The closure type is not an aggregate type.
An implementation may define the closure type differently from what is described below provided this does not alter the observable behavior of the program other than by changing:
An implementation shall not add members of rvalue reference type to the closure type.
3
#
The closure type for a lambda-expression has a public inline function call operator (for a non-generic lambda) or function call operator template (for a generic lambda) ([over.call]) whose parameters and return type are described by the lambda-expression's parameter-declaration-clause and trailing-return-type respectively, and whose template-parameter-list consists of the specified template-parameter-list, if any.
The requires-clause of the function call operator template is the requires-clause immediately following < template-parameter-list >, if any.
The trailing requires-clause of the function call operator or operator template is the requires-clause of the lambda-declarator, if any.
[Note 2:
The function call operator template for a generic lambda might be an abbreviated function template ([dcl.fct]).
— end note]
[Example 1: auto glambda = [](auto a, auto&& b) { return a < b; }; bool b = glambda(3, 3.14); // OK auto vglambda = [](auto printer) { return [=](auto&& ... ts) { // OK: ts is a function parameter pack printer(std::forward<decltype(ts)>(ts)...); return [=]() { printer(ts ...); }; }; }; auto p = vglambda( [](auto v1, auto v2, auto v3) { std::cout << v1 << v2 << v3; } ); auto q = p(1, 'a', 3.14); // OK: outputs 1a3.14 q(); // OK: outputs 1a3.14 — end example]
4
#
The function call operator or operator template is declared const ([class.mfct.non-static]) if and only if the lambda-expression's parameter-declaration-clause is not followed by mutable.
It is neither virtual nor declared volatile.
Any noexcept-specifier specified on a lambda-expression applies to the corresponding function call operator or operator template.
An attribute-specifier-seq in a lambda-declarator appertains to the type of the corresponding function call operator or operator template.
The function call operator or any given operator template specialization is a constexpr function if either the corresponding lambda-expression's parameter-declaration-clause is followed by constexpr or consteval, or it satisfies the requirements for a constexpr function ([dcl.constexpr]).
It is an immediate function ([dcl.constexpr]) if the corresponding lambda-expression's parameter-declaration-clause is followed by consteval.
[Note 3:
Names referenced in the lambda-declarator are looked up in the context in which the lambda-expression appears.
— end note]
[Example 2: auto ID = [](auto a) { return a; }; static_assert(ID(3) == 3); // OK struct NonLiteral { NonLiteral(int n) : n(n) { } int n; }; static_assert(ID(NonLiteral{3}).n == 3); // error — end example]
5
#
[Example 3: auto monoid = [](auto v) { return [=] { return v; }; }; auto add = [](auto m1) constexpr { auto ret = m1(); return [=](auto m2) mutable { auto m1val = m1(); auto plus = [=](auto m2val) mutable constexpr { return m1val += m2val; }; ret = plus(m2()); return monoid(ret); }; }; constexpr auto zero = monoid(0); constexpr auto one = monoid(1); static_assert(add(one)(zero)() == one()); // OK // Since two below is not declared constexpr, an evaluation of its constexpr member function call operator // cannot perform an lvalue-to-rvalue conversion on one of its subobjects (that represents its capture) // in a constant expression. auto two = monoid(2); assert(two() == 2); // OK, not a constant expression. static_assert(add(one)(one)() == two()); // error: two() is not a constant expression static_assert(add(one)(one)() == monoid(2)()); // OK — end example]
6
#
[Note 4:
The function call operator or operator template can be constrained ([temp.constr.decl]) by a type-constraint ([temp.param]), a requires-clause ([temp.pre]), or a trailing requires-clause ([dcl.decl]).
[Example 4: template <typename T> concept C1 = /* ... */; template <std::size_t N> concept C2 = /* ... */; template <typename A, typename B> concept C3 = /* ... */; auto f = []<typename T1, C1 T2> requires C2<sizeof(T1) + sizeof(T2)> (T1 a1, T1 b1, T2 a2, auto a3, auto a4) requires C3<decltype(a4), T2> { // T2 is constrained by a type-constraint. // T1 and T2 are constrained by a requires-clause, and // T2 and the type of a4 are constrained by a trailing requires-clause. }; — end example]
— end note]
7
#
The closure type for a non-generic lambda-expression with no lambda-capture whose constraints (if any) are satisfied has a conversion function to pointer to function with C++ language linkage having the same parameter and return types as the closure type's function call operator.
The conversion is to “pointer to noexcept function” if the function call operator has a non-throwing exception specification.
The value returned by this conversion function is the address of a function F that, when invoked, has the same effect as invoking the closure type's function call operator on a default-constructed instance of the closure type.
F is a constexpr function if the function call operator is a constexpr function and is an immediate function if the function call operator is an immediate function.
8
#
For a generic lambda with no lambda-capture, the closure type has a conversion function template to pointer to function.
The conversion function template has the same invented template parameter list, and the pointer to function has the same parameter types, as the function call operator template.
The return type of the pointer to function shall behave as if it were a decltype-specifier denoting the return type of the corresponding function call operator template specialization.
9
#
[Note 5:
If the generic lambda has no trailing-return-type or the trailing-return-type contains a placeholder type, return type deduction of the corresponding function call operator template specialization has to be done.
The corresponding specialization is that instantiation of the function call operator template with the same template arguments as those deduced for the conversion function template.
Consider the following: auto glambda = [](auto a) { return a; }; int (*fp)(int) = glambda;
The behavior of the conversion function of glambda above is like that of the following conversion function: struct Closure { template<class T> auto operator()(T t) const { /* ... */ } template<class T> static auto lambda_call_operator_invoker(T a) { // forwards execution to operator()(a) and therefore has // the same return type deduced /* ... */ } template<class T> using fptr_t = decltype(lambda_call_operator_invoker(declval<T>())) (*)(T); template<class T> operator fptr_t<T>() const { return &lambda_call_operator_invoker; } };
— end note]
[Example 5: void f1(int (*)(int)) { } void f2(char (*)(int)) { } void g(int (*)(int)) { } // #1 void g(char (*)(char)) { } // #2 void h(int (*)(int)) { } // #3 void h(char (*)(int)) { } // #4 auto glambda = [](auto a) { return a; }; f1(glambda); // OK f2(glambda); // error: ID is not convertible g(glambda); // error: ambiguous h(glambda); // OK: calls #3 since it is convertible from ID int& (*fpi)(int*) = [](auto* a) -> auto& { return *a; }; // OK — end example]
10
#
The value returned by any given specialization of this conversion function template is the address of a function F that, when invoked, has the same effect as invoking the generic lambda's corresponding function call operator template specialization on a default-constructed instance of the closure type.
F is a constexpr function if the corresponding specialization is a constexpr function and F is an immediate function if the function call operator template specialization is an immediate function.
[Note 6:
This will result in the implicit instantiation of the generic lambda's body.
The instantiated generic lambda's return type and parameter types are required to match the return type and parameter types of the pointer to function.
— end note]
[Example 6: auto GL = [](auto a) { std::cout << a; return a; }; int (*GL_int)(int) = GL; // OK: through conversion function template GL_int(3); // OK: same as GL(3) — end example]
11
#
The conversion function or conversion function template is public, constexpr, non-virtual, non-explicit, const, and has a non-throwing exception specification.
[Example 7: auto Fwd = [](int (*fp)(int), auto a) { return fp(a); }; auto C = [](auto a) { return a; }; static_assert(Fwd(C,3) == 3); // OK // No specialization of the function call operator template can be constexpr (due to the local static). auto NC = [](auto a) { static int s; return a; }; static_assert(Fwd(NC,3) == 3); // error — end example]
12
#
The lambda-expression's compound-statement yields the function-body ([dcl.fct.def]) of the function call operator, but for purposes of name lookup, determining the type and value of this and transforming id-expressions referring to non-static class members into class member access expressions using (*this) ([class.mfct.non-static]), the compound-statement is considered in the context of the lambda-expression.
[Example 8: struct S1 { int x, y; int operator()(int); void f() { [=]()->int { return operator()(this->x + y); // equivalent to S1​::​operator()(this->x + (*this).y) // this has type S1* }; } }; — end example]
Further, a variable __func__ is implicitly defined at the beginning of the compound-statement of the lambda-expression, with semantics as described in [dcl.fct.def.general].
13
#
The closure type associated with a lambda-expression has no default constructor if the lambda-expression has a lambda-capture and a defaulted default constructor otherwise.
It has a defaulted copy constructor and a defaulted move constructor ([class.copy.ctor]).
It has a deleted copy assignment operator if the lambda-expression has a lambda-capture and defaulted copy and move assignment operators otherwise ([class.copy.assign]).
[Note 7:
These special member functions are implicitly defined as usual, and might therefore be defined as deleted.
— end note]
14
#
The closure type associated with a lambda-expression has an implicitly-declared destructor ([class.dtor]).
15
#
A member of a closure type shall not be explicitly instantiated, explicitly specialized, or named in a friend declaration.

7.5.5.3 Captures [expr.prim.lambda.capture]

simple-capture:
identifier ...
& identifier ...
this
* this
1
#
The body of a lambda-expression may refer to local entities of enclosing block scopes by capturing those entities, as described below.
2
#
If a lambda-capture includes a capture-default that is &, no identifier in a simple-capture of that lambda-capture shall be preceded by &.
If a lambda-capture includes a capture-default that is =, each simple-capture of that lambda-capture shall be of the form “& identifier ...”, “this”, or “* this”.
[Note 1:
The form [&,this] is redundant but accepted for compatibility with ISO C++ 2014.
— end note]
Ignoring appearances in initializers of init-captures, an identifier or this shall not appear more than once in a lambda-capture.
[Example 1: struct S2 { void f(int i); }; void S2::f(int i) { [&, i]{ }; // OK [&, this, i]{ }; // OK, equivalent to [&, i] [&, &i]{ }; // error: i preceded by & when & is the default [=, *this]{ }; // OK [=, this]{ }; // OK, equivalent to [=] [i, i]{ }; // error: i repeated [this, *this]{ }; // error: this appears twice } — end example]
3
#
A lambda-expression shall not have a capture-default or simple-capture in its lambda-introducer unless its innermost enclosing scope is a block scope ([basic.scope.block]) or it appears within a default member initializer and its innermost enclosing scope is the corresponding class scope ([basic.scope.class]).
4
#
The identifier in a simple-capture is looked up using the usual rules for unqualified name lookup; each such lookup shall find a local entity.
The simple-captures this and * this denote the local entity *this.
An entity that is designated by a simple-capture is said to be explicitly captured.
5
#
If an identifier in a simple-capture appears as the declarator-id of a parameter of the lambda-declarator's parameter-declaration-clause, the program is ill-formed.
[Example 2: void f() { int x = 0; auto g = [x](int x) { return 0; }; // error: parameter and simple-capture have the same name } — end example]
6
#
An init-capture without ellipsis behaves as if it declares and explicitly captures a variable of the form “auto init-capture ;” whose declarative region is the lambda-expression's compound-statement, except that:
  • (6.1)
    if the capture is by copy (see below), the non-static data member declared for the capture and the variable are treated as two different ways of referring to the same object, which has the lifetime of the non-static data member, and no additional copy and destruction is performed, and
  • (6.2)
    if the capture is by reference, the variable's lifetime ends when the closure object's lifetime ends.
[Note 2:
This enables an init-capture like “x = std​::​move(x)”; the second “x” must bind to a declaration in the surrounding context.
— end note]
[Example 3: int x = 4; auto y = [&r = x, x = x+1]()->int { r += 2; return x+2; }(); // Updates ​::​x to 6, and initializes y to 7. auto z = [a = 42](int a) { return 1; }; // error: parameter and local variable have the same name — end example]
7
#
For the purposes of lambda capture, an expression potentially references local entities as follows:
If an expression potentially references a local entity within a declarative region in which it is odr-usable, and the expression would be potentially evaluated if the effect of any enclosing typeid expressions ([expr.typeid]) were ignored, the entity is said to be implicitly captured by each intervening lambda-expression with an associated capture-default that does not explicitly capture it.
The implicit capture of *this is deprecated when the capture-default is =; see [depr.capture.this].
[Example 4: void f(int, const int (&)[2] = {}); // #1 void f(const int&, const int (&)[1]); // #2 void test() { const int x = 17; auto g = [](auto a) { f(x); // OK: calls #1, does not capture x }; auto g1 = [=](auto a) { f(x); // OK: calls #1, captures x }; auto g2 = [=](auto a) { int selector[sizeof(a) == 1 ? 1 : 2]{}; f(x, selector); // OK: captures x, might call #1 or #2 }; auto g3 = [=](auto a) { typeid(a + x); // captures x regardless of whether a + x is an unevaluated operand }; }
Within g1, an implementation might optimize away the capture of x as it is not odr-used.
— end example]
[Note 4:
The set of captured entities is determined syntactically, and entities might be implicitly captured even if the expression denoting a local entity is within a discarded statement ([stmt.if]).
[Example 5: template<bool B> void f(int n) { [=](auto a) { if constexpr (B && sizeof(a) > 4) { (void)n; // captures n regardless of the value of B and sizeof(int) } }(0); } — end example]
— end note]
8
#
An entity is captured if it is captured explicitly or implicitly.
An entity captured by a lambda-expression is odr-used ([basic.def.odr]) in the scope containing the lambda-expression.
[Note 5:
As a consequence, if a lambda-expression explicitly captures an entity that is not odr-usable, the program is ill-formed ([basic.def.odr]).
— end note]
[Example 6: void f1(int i) { int const N = 20; auto m1 = [=]{ int const M = 30; auto m2 = [i]{ int x[N][M]; // OK: N and M are not odr-used x[0][0] = i; // OK: i is explicitly captured by m2 and implicitly captured by m1 }; }; struct s1 { int f; void work(int n) { int m = n*n; int j = 40; auto m3 = [this,m] { auto m4 = [&,j] { // error: j not odr-usable due to intervening lambda m3 int x = n; // error: n is odr-used but not odr-usable due to intervening lambda m3 x += m; // OK: m implicitly captured by m4 and explicitly captured by m3 x += i; // error: i is odr-used but not odr-usable // due to intervening function and class scopes x += f; // OK: this captured implicitly by m4 and explicitly by m3 }; }; } }; } struct s2 { double ohseven = .007; auto f() { return [this] { return [*this] { return ohseven; // OK }; }(); } auto g() { return [] { return [*this] { }; // error: *this not captured by outer lambda-expression }(); } }; — end example]
9
#
[Note 6:
Because local entities are not odr-usable within a default argument ([basic.def.odr]), a lambda-expression appearing in a default argument cannot implicitly or explicitly capture any local entity.
Such a lambda-expression can still have an init-capture if any full-expression in its initializer satisfies the constraints of an expression appearing in a default argument ([dcl.fct.default]).
— end note]
[Example 7: void f2() { int i = 1; void g1(int = ([i]{ return i; })()); // error void g2(int = ([i]{ return 0; })()); // error void g3(int = ([=]{ return i; })()); // error void g4(int = ([=]{ return 0; })()); // OK void g5(int = ([]{ return sizeof i; })()); // OK void g6(int = ([x=1]{ return x; })()); // OK void g7(int = ([x=i]{ return x; })()); // error } — end example]
10
#
An entity is captured by copy if
For each entity captured by copy, an unnamed non-static data member is declared in the closure type.
The declaration order of these members is unspecified.
The type of such a data member is the referenced type if the entity is a reference to an object, an lvalue reference to the referenced function type if the entity is a reference to a function, or the type of the corresponding captured entity otherwise.
A member of an anonymous union shall not be captured by copy.
11
#
Every id-expression within the compound-statement of a lambda-expression that is an odr-use of an entity captured by copy is transformed into an access to the corresponding unnamed data member of the closure type.
[Note 7:
An id-expression that is not an odr-use refers to the original entity, never to a member of the closure type.
However, such an id-expression can still cause the implicit capture of the entity.
— end note]
If *this is captured by copy, each expression that odr-uses *this is transformed to instead refer to the corresponding unnamed data member of the closure type.
[Example 8: void f(const int*); void g() { const int N = 10; [=] { int arr[N]; // OK: not an odr-use, refers to automatic variable f(&N); // OK: causes N to be captured; &N points to // the corresponding member of the closure type }; } — end example]
12
#
An entity is captured by reference if it is implicitly or explicitly captured but not captured by copy.
It is unspecified whether additional unnamed non-static data members are declared in the closure type for entities captured by reference.
If declared, such non-static data members shall be of literal type.
[Example 9: // The inner closure type must be a literal type regardless of how reference captures are represented. static_assert([](int n) { return [&n] { return ++n; }(); }(3) == 4); — end example]
A bit-field or a member of an anonymous union shall not be captured by reference.
13
#
An id-expression within the compound-statement of a lambda-expression that is an odr-use of a reference captured by reference refers to the entity to which the captured reference is bound and not to the captured reference.
[Note 8:
The validity of such captures is determined by the lifetime of the object to which the reference refers, not by the lifetime of the reference itself.
— end note]
[Example 10: auto h(int &r) { return [&] { ++r; // Valid after h returns if the lifetime of the // object to which r is bound has not ended }; } — end example]
14
#
If a lambda-expression m2 captures an entity and that entity is captured by an immediately enclosing lambda-expression m1, then m2's capture is transformed as follows:
  • (14.1)
    if m1 captures the entity by copy, m2 captures the corresponding non-static data member of m1's closure type;
  • (14.2)
    if m1 captures the entity by reference, m2 captures the same entity captured by m1.
[Example 11:
The nested lambda-expressions and invocations below will output 123234.
int a = 1, b = 1, c = 1; auto m1 = [a, &b, &c]() mutable { auto m2 = [a, b, &c]() mutable { std::cout << a << b << c; a = 4; b = 4; c = 4; }; a = 3; b = 3; c = 3; m2(); }; a = 2; b = 2; c = 2; m1(); std::cout << a << b << c; — end example]
15
#
When the lambda-expression is evaluated, the entities that are captured by copy are used to direct-initialize each corresponding non-static data member of the resulting closure object, and the non-static data members corresponding to the init-captures are initialized as indicated by the corresponding initializer (which may be copy- or direct-initialization).
(For array members, the array elements are direct-initialized in increasing subscript order.)
These initializations are performed in the (unspecified) order in which the non-static data members are declared.
[Note 9:
This ensures that the destructions will occur in the reverse order of the constructions.
— end note]
16
#
[Note 10:
If a non-reference entity is implicitly or explicitly captured by reference, invoking the function call operator of the corresponding lambda-expression after the lifetime of the entity has ended is likely to result in undefined behavior.
— end note]
17
#
A simple-capture containing an ellipsis is a pack expansion ([temp.variadic]).
An init-capture containing an ellipsis is a pack expansion that introduces an init-capture pack ([temp.variadic]) whose declarative region is the lambda-expression's compound-statement.
[Example 12: template<class... Args> void f(Args... args) { auto lm = [&, args...] { return g(args...); }; lm(); auto lm2 = [...xs=std::move(args)] { return g(xs...); }; lm2(); } — end example]

7.5.6 Fold expressions [expr.prim.fold]

1
#
A fold expression performs a fold of a pack ([temp.variadic]) over a binary operator.
fold-operator: one of
+   -   *   /   %   ^   &   |   <<   >>
+=  -=  *=  /=  %=  ^=  &=  |=  <<=  >>=  =
==  !=  <   >   <=  >=  &&  ||  ,   .*   ->*
2
#
An expression of the form (... op e) where op is a fold-operator is called a unary left fold.
An expression of the form (e op ...) where op is a fold-operator is called a unary right fold.
Unary left folds and unary right folds are collectively called unary folds.
In a unary fold, the cast-expression shall contain an unexpanded pack ([temp.variadic]).
3
#
An expression of the form (e1 op1 ... op2 e2) where op1 and op2 are fold-operators is called a binary fold.
In a binary fold, op1 and op2 shall be the same fold-operator, and either e1 shall contain an unexpanded pack or e2 shall contain an unexpanded pack, but not both.
If e2 contains an unexpanded pack, the expression is called a binary left fold.
If e1 contains an unexpanded pack, the expression is called a binary right fold.
[Example 1: template<typename ...Args> bool f(Args ...args) { return (true && ... && args); // OK } template<typename ...Args> bool f(Args ...args) { return (args + ... + args); // error: both operands contain unexpanded packs } — end example]

7.5.7 Requires expressions [expr.prim.req]

7.5.7.1 General [expr.prim.req.general]

2
#
A requires-expression is a prvalue of type bool whose value is described below.
Expressions appearing within a requirement-body are unevaluated operands.
3
#
[Example 1:
A common use of requires-expressions is to define requirements in concepts such as the one below: template<typename T> concept R = requires (T i) { typename T::type; {*i} -> std::convertible_­to<const typename T::type&>; };
A requires-expression can also be used in a requires-clause ([temp.pre]) as a way of writing ad hoc constraints on template arguments such as the one below: template<typename T> requires requires (T x) { x + x; } T add(T a, T b) { return a + b; }
The first requires introduces the requires-clause, and the second introduces the requires-expression.
— end example]
4
#
A requires-expression may introduce local parameters using a parameter-declaration-clause ([dcl.fct]).
A local parameter of a requires-expression shall not have a default argument.
Each name introduced by a local parameter is in scope from the point of its declaration until the closing brace of the requirement-body.
These parameters have no linkage, storage, or lifetime; they are only used as notation for the purpose of defining requirements.
The parameter-declaration-clause of a requirement-parameter-list shall not terminate with an ellipsis.
[Example 2: template<typename T> concept C = requires(T t, ...) { // error: terminates with an ellipsis t; }; — end example]
5
#
The requirement-body contains a sequence of requirements.
These requirements may refer to local parameters, template parameters, and any other declarations visible from the enclosing context.
6
#
The substitution of template arguments into a requires-expression may result in the formation of invalid types or expressions in its requirements or the violation of the semantic constraints of those requirements.
In such cases, the requires-expression evaluates to false; it does not cause the program to be ill-formed.
The substitution and semantic constraint checking proceeds in lexical order and stops when a condition that determines the result of the requires-expression is encountered.
If substitution (if any) and semantic constraint checking succeed, the requires-expression evaluates to true.
[Note 1:
If a requires-expression contains invalid types or expressions in its requirements, and it does not appear within the declaration of a templated entity, then the program is ill-formed.
— end note]
If the substitution of template arguments into a requirement would always result in a substitution failure, the program is ill-formed; no diagnostic required.
[Example 3: template<typename T> concept C = requires { new int[-(int)sizeof(T)]; // ill-formed, no diagnostic required }; — end example]

7.5.7.2 Simple requirements [expr.prim.req.simple]

1
#
A simple-requirement asserts the validity of an expression.
[Note 1:
The enclosing requires-expression will evaluate to false if substitution of template arguments into the expression fails.
— end note]
[Example 1: template<typename T> concept C = requires (T a, T b) { a + b; // C<T> is true if a + b is a valid expression }; — end example]
2
#
A requirement that starts with a requires token is never interpreted as a simple-requirement.
[Note 2:
This simplifies distinguishing between a simple-requirement and a nested-requirement.
— end note]

7.5.7.3 Type requirements [expr.prim.req.type]

1
#
A type-requirement asserts the validity of a type.
[Note 1:
The enclosing requires-expression will evaluate to false if substitution of template arguments fails.
— end note]
[Example 1: template<typename T, typename T::type = 0> struct S; template<typename T> using Ref = T&; template<typename T> concept C = requires { typename T::inner; // required nested member name typename S<T>; // required class template specialization typename Ref<T>; // required alias template substitution, fails if T is void }; — end example]
2
#
A type-requirement that names a class template specialization does not require that type to be complete ([basic.types]).

7.5.7.4 Compound requirements [expr.prim.req.compound]

1
#
A compound-requirement asserts properties of the expression E.
Substitution of template arguments (if any) and verification of semantic properties proceed in the following order:
  • (1.1)
    Substitution of template arguments (if any) into the expression is performed.
  • (1.2)
    If the noexcept specifier is present, E shall not be a potentially-throwing expression ([except.spec]).
  • (1.3)
    If the return-type-requirement is present, then:
    • (1.3.1)
      Substitution of template arguments (if any) into the return-type-requirement is performed.
    • (1.3.2)
      The immediately-declared constraint ([temp.param]) of the type-constraint for decltype((E)) shall be satisfied.
      [Example 1:
      Given concepts C and D, requires { { E1 } -> C; { E2 } -> D<A, ⋯, A>; }; is equivalent to requires { E1; requires C<decltype((E1))>; E2; requires D<decltype((E2)), A, ⋯, A>; }; (including in the case where n is zero).
      — end example]
2
#
[Example 2: template<typename T> concept C1 = requires(T x) { {x++}; };
The compound-requirement in C1 requires that x++ is a valid expression.
It is equivalent to the simple-requirement x++;.
template<typename T> concept C2 = requires(T x) { {*x} -> std::same_­as<typename T::inner>; };
The compound-requirement in C2 requires that *x is a valid expression, that typename T​::​inner is a valid type, and that std​::​same_­as<decltype((*x)), typename T​::​inner> is satisfied.
template<typename T> concept C3 = requires(T x) { {g(x)} noexcept; };
The compound-requirement in C3 requires that g(x) is a valid expression and that g(x) is non-throwing.
— end example]

7.5.7.5 Nested requirements [expr.prim.req.nested]

1
#
A nested-requirement can be used to specify additional constraints in terms of local parameters.
The constraint-expression shall be satisfied ([temp.constr.decl]) by the substituted template arguments, if any.
Substitution of template arguments into a nested-requirement does not result in substitution into the constraint-expression other than as specified in [temp.constr.constr].
[Example 1:
template<typename U> concept C = sizeof(U) == 1; template<typename T> concept D = requires (T t) { requires C<decltype (+t)>; }; D<T> is satisfied if sizeof(decltype (+t)) == 1 ([temp.constr.atomic]).
— end example]
2
#
A local parameter shall only appear as an unevaluated operand within the constraint-expression.
[Example 2: template<typename T> concept C = requires (T a) { requires sizeof(a) == 4; // OK requires a == 0; // error: evaluation of a constraint variable }; — end example]

7.6 Compound expressions [expr.compound]

7.6.1 Postfix expressions [expr.post]

7.6.1.1 General [expr.post.general]

2
#
[Note 1:
The > token following the type-id in a dynamic_­cast, static_­cast, reinterpret_­cast, or const_­cast might be the product of replacing a >> token by two consecutive > tokens ([temp.names]).
— end note]

7.6.1.2 Subscripting [expr.sub]

1
#
A postfix expression followed by an expression in square brackets is a postfix expression.
One of the expressions shall be a glvalue of type “array of T” or a prvalue of type “pointer to T” and the other shall be a prvalue of unscoped enumeration or integral type.
The result is of type “T”.
The type “T” shall be a completely-defined object type.62
The expression E1[E2] is identical (by definition) to *((E1)+(E2)), except that in the case of an array operand, the result is an lvalue if that operand is an lvalue and an xvalue otherwise.
The expression E1 is sequenced before the expression E2.
2
#
[Note 1:
A comma expression appearing as the expr-or-braced-init-list of a subscripting expression is deprecated; see [depr.comma.subscript].
— end note]
3
#
[Note 2:
Despite its asymmetric appearance, subscripting is a commutative operation except for sequencing.
See [expr.unary] and [expr.add] for details of * and + and [dcl.array] for details of array types.
— end note]
4
#
A braced-init-list shall not be used with the built-in subscript operator.
62)
This is true even if the subscript operator is used in the following common idiom: &x[0].
 ⮥

7.6.1.3 Function call [expr.call]

1
#
A function call is a postfix expression followed by parentheses containing a possibly empty, comma-separated list of initializer-clauses which constitute the arguments to the function.
[Note 1:
If the postfix expression is a function or member function name, the appropriate function and the validity of the call are determined according to the rules in [over.match].
— end note]
The postfix expression shall have function type or function pointer type.
For a call to a non-member function or to a static member function, the postfix expression shall either be an lvalue that refers to a function (in which case the function-to-pointer standard conversion ([conv.func]) is suppressed on the postfix expression), or have function pointer type.
2
#
For a call to a non-static member function, the postfix expression shall be an implicit ([class.mfct.non-static], [class.static]) or explicit class member access whose id-expression is a function member name, or a pointer-to-member expression selecting a function member; the call is as a member of the class object referred to by the object expression.
In the case of an implicit class member access, the implied object is the one pointed to by this.
[Note 2:
A member function call of the form f() is interpreted as (*this).f() (see [class.mfct.non-static]).
— end note]
3
#
If the selected function is non-virtual, or if the id-expression in the class member access expression is a qualified-id, that function is called.
Otherwise, its final overrider in the dynamic type of the object expression is called; such a call is referred to as a virtual function call.
[Note 3:
The dynamic type is the type of the object referred to by the current value of the object expression.
[class.cdtor] describes the behavior of virtual function calls when the object expression refers to an object under construction or destruction.
— end note]
4
#
[Note 4:
If a function or member function name is used, and name lookup does not find a declaration of that name, the program is ill-formed.
No function is implicitly declared by such a call.
— end note]
5
#
If the postfix-expression names a destructor or pseudo-destructor ([expr.prim.id.dtor]), the type of the function call expression is void; otherwise, the type of the function call expression is the return type of the statically chosen function (i.e., ignoring the virtual keyword), even if the type of the function actually called is different.
This return type shall be an object type, a reference type or cv void.
If the postfix-expression names a pseudo-destructor (in which case the postfix-expression is a possibly-parenthesized class member access), the function call destroys the object of scalar type denoted by the object expression of the class member access ([expr.ref], [basic.life]).
6
#
Calling a function through an expression whose function type is different from the function type of the called function's definition results in undefined behavior.
7
#
When a function is called, each parameter ([dcl.fct]) is initialized ([dcl.init], [class.copy.ctor]) with its corresponding argument.
If there is no corresponding argument, the default argument for the parameter is used.
[Example 1: template<typename ...T> int f(int n = 0, T ...t); int x = f<int>(); // error: no argument for second function parameter — end example]
If the function is a non-static member function, the this parameter of the function is initialized with a pointer to the object of the call, converted as if by an explicit type conversion.
[Note 5:
There is no access or ambiguity checking on this conversion; the access checking and disambiguation are done as part of the (possibly implicit) class member access operator.
— end note]
When a function is called, the type of any parameter shall not be a class type that is either incomplete or abstract.
[Note 6:
This still allows a parameter to be a pointer or reference to such a type.
However, it prevents a passed-by-value parameter to have an incomplete or abstract class type.
— end note]
It is implementation-defined whether the lifetime of a parameter ends when the function in which it is defined returns or at the end of the enclosing full-expression.
The initialization and destruction of each parameter occurs within the context of the calling function.
[Example 2:
The access of the constructor, conversion functions or destructor is checked at the point of call in the calling function.
If a constructor or destructor for a function parameter throws an exception, the search for a handler starts in the scope of the calling function; in particular, if the function called has a function-try-block ([except.pre]) with a handler that could handle the exception, this handler is not considered.
— end example]
8
#
The postfix-expression is sequenced before each expression in the expression-list and any default argument.
The initialization of a parameter, including every associated value computation and side effect, is indeterminately sequenced with respect to that of any other parameter.
[Note 7:
All side effects of argument evaluations are sequenced before the function is entered (see [intro.execution]).
— end note]
[Example 3: void f() { std::string s = "but I have heard it works even if you don't believe in it"; s.replace(0, 4, "").replace(s.find("even"), 4, "only").replace(s.find(" don't"), 6, ""); assert(s == "I have heard it works only if you believe in it"); // OK } — end example]
[Note 8:
If an operator function is invoked using operator notation, argument evaluation is sequenced as specified for the built-in operator; see [over.match.oper].
— end note]
[Example 4: struct S { S(int); }; int operator<<(S, int); int i, j; int x = S(i=1) << (i=2); int y = operator<<(S(j=1), j=2);
After performing the initializations, the value of i is 2 (see [expr.shift]), but it is unspecified whether the value of j is 1 or 2.
— end example]
9
#
The result of a function call is the result of the possibly-converted operand of the return statement that transferred control out of the called function (if any), except in a virtual function call if the return type of the final overrider is different from the return type of the statically chosen function, the value returned from the final overrider is converted to the return type of the statically chosen function.
10
#
[Note 9:
A function can change the values of its non-const parameters, but these changes cannot affect the values of the arguments except where a parameter is of a reference type ([dcl.ref]); if the reference is to a const-qualified type, const_­cast is required to be used to cast away the constness in order to modify the argument's value.
Where a parameter is of const reference type a temporary object is introduced if needed ([dcl.type], [lex.literal], [lex.string], [dcl.array], [class.temporary]).
In addition, it is possible to modify the values of non-constant objects through pointer parameters.
— end note]
11
#
A function can be declared to accept fewer arguments (by declaring default arguments) or more arguments (by using the ellipsis, ..., or a function parameter pack ([dcl.fct])) than the number of parameters in the function definition.
[Note 10:
This implies that, except where the ellipsis (...) or a function parameter pack is used, a parameter is available for each argument.
— end note]
12
#
When there is no parameter for a given argument, the argument is passed in such a way that the receiving function can obtain the value of the argument by invoking va_­arg.
[Note 11:
This paragraph does not apply to arguments passed to a function parameter pack.
Function parameter packs are expanded during template instantiation ([temp.variadic]), thus each such argument has a corresponding parameter when a function template specialization is actually called.
— end note]
The lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the argument expression.
An argument that has type cv std​::​nullptr_­t is converted to type void*.
After these conversions, if the argument does not have arithmetic, enumeration, pointer, pointer-to-member, or class type, the program is ill-formed.
Passing a potentially-evaluated argument of a scoped enumeration type or of a class type ([class]) having an eligible non-trivial copy constructor, an eligible non-trivial move constructor, or a non-trivial destructor ([special]), with no corresponding parameter, is conditionally-supported with implementation-defined semantics.
If the argument has integral or enumeration type that is subject to the integral promotions, or a floating-point type that is subject to the floating-point promotion, the value of the argument is converted to the promoted type before the call.
These promotions are referred to as the default argument promotions.
13
#
Recursive calls are permitted, except to the main function.
14
#
A function call is an lvalue if the result type is an lvalue reference type or an rvalue reference to function type, an xvalue if the result type is an rvalue reference to object type, and a prvalue otherwise.

7.6.1.4 Explicit type conversion (functional notation) [expr.type.conv]

1
#
A simple-type-specifier or typename-specifier followed by a parenthesized optional expression-list or by a braced-init-list (the initializer) constructs a value of the specified type given the initializer.
If the type is a placeholder for a deduced class type, it is replaced by the return type of the function selected by overload resolution for class template deduction for the remainder of this subclause.
2
#
If the initializer is a parenthesized single expression, the type conversion expression is equivalent to the corresponding cast expression.
Otherwise, if the type is cv void and the initializer is () or {} (after pack expansion, if any), the expression is a prvalue of the specified type that performs no initialization.
Otherwise, the expression is a prvalue of the specified type whose result object is direct-initialized with the initializer.
If the initializer is a parenthesized optional expression-list, the specified type shall not be an array type.

7.6.1.5 Class member access [expr.ref]

1
#
A postfix expression followed by a dot . or an arrow ->, optionally followed by the keyword template ([temp.names]), and then followed by an id-expression, is a postfix expression.
The postfix expression before the dot or arrow is evaluated;63 the result of that evaluation, together with the id-expression, determines the result of the entire postfix expression.
2
#
For the first option (dot) the first expression shall be a glvalue.
For the second option (arrow) the first expression shall be a prvalue having pointer type.
The expression E1->E2 is converted to the equivalent form (*(E1)).E2; the remainder of [expr.ref] will address only the first option (dot).64
3
#
Abbreviating postfix-expression.id-expression as E1.E2, E1 is called the object expression.
If the object expression is of scalar type, E2 shall name the pseudo-destructor of that same type (ignoring cv-qualifications) and E1.E2 is an lvalue of type “function of () returning void”.
[Note 1:
This value can only be used for a notional function call ([expr.prim.id.dtor]).
— end note]
4
#
Otherwise, the object expression shall be of class type.
The class type shall be complete unless the class member access appears in the definition of that class.
[Note 2:
If the class is incomplete, lookup in the complete class type is required to refer to the same declaration ([basic.scope.class]).
— end note]
The id-expression shall name a member of the class or of one of its base classes.
[Note 3:
Because the name of a class is inserted in its class scope ([class]), the name of a class is also considered a nested member of that class.
— end note]
[Note 4:
[basic.lookup.classref] describes how names are looked up after the . and -> operators.
— end note]
5
#
If E2 is a bit-field, E1.E2 is a bit-field.
The type and value category of E1.E2 are determined as follows.
In the remainder of [expr.ref], cq represents either const or the absence of const and vq represents either volatile or the absence of volatile.
cv represents an arbitrary set of cv-qualifiers, as defined in [basic.type.qualifier].
6
#
If E2 is declared to have type “reference to T”, then E1.E2 is an lvalue; the type of E1.E2 is T.
Otherwise, one of the following rules applies.
  • (6.1)
    If E2 is a static data member and the type of E2 is T, then E1.E2 is an lvalue; the expression designates the named member of the class.
    The type of E1.E2 is T.
  • (6.2)
    If E2 is a non-static data member and the type of E1 is “cq1 vq1 X”, and the type of E2 is “cq2 vq2 T”, the expression designates the corresponding member subobject of the object designated by the first expression.
    If E1 is an lvalue, then E1.E2 is an lvalue; otherwise E1.E2 is an xvalue.
    Let the notation vq12 stand for the “union” of vq1 and vq2; that is, if vq1 or vq2 is volatile, then vq12 is volatile.
    Similarly, let the notation cq12 stand for the “union” of cq1 and cq2; that is, if cq1 or cq2 is const, then cq12 is const.
    If E2 is declared to be a mutable member, then the type of E1.E2 is “vq12 T”.
    If E2 is not declared to be a mutable member, then the type of E1.E2 is “cq12 vq12 T”.
  • (6.3)
    If E2 is a (possibly overloaded) member function, function overload resolution ([over.match]) is used to select the function to which E2 refers.
    The type of E1.E2 is the type of E2 and E1.E2 refers to the function referred to by E2.
    • (6.3.1)
      If E2 refers to a static member function, E1.E2 is an lvalue.
    • (6.3.2)
      Otherwise (when E2 refers to a non-static member function), E1.E2 is a prvalue.
      The expression can be used only as the left-hand operand of a member function call ([class.mfct]).
      [Note 5:
      Any redundant set of parentheses surrounding the expression is ignored ([expr.prim.paren]).
      — end note]
  • (6.4)
    If E2 is a nested type, the expression E1.E2 is ill-formed.
  • (6.5)
    If E2 is a member enumerator and the type of E2 is T, the expression E1.E2 is a prvalue.
    The type of E1.E2 is T.
7
#
If E2 is a non-static data member or a non-static member function, the program is ill-formed if the class of which E2 is directly a member is an ambiguous base ([class.member.lookup]) of the naming class ([class.access.base]) of E2.
[Note 6:
The program is also ill-formed if the naming class is an ambiguous base of the class type of the object expression; see [class.access.base].
— end note]
63)
If the class member access expression is evaluated, the subexpression evaluation happens even if the result is unnecessary to determine the value of the entire postfix expression, for example if the id-expression denotes a static member.
 ⮥
64)
Note that (*(E1)) is an lvalue.
 ⮥

7.6.1.6 Increment and decrement [expr.post.incr]

1
#
The value of a postfix ++ expression is the value of its operand.
[Note 1:
The value obtained is a copy of the original value.
— end note]
The operand shall be a modifiable lvalue.
The type of the operand shall be an arithmetic type other than cv bool, or a pointer to a complete object type.
An operand with volatile-qualified type is deprecated; see [depr.volatile.type].
The value of the operand object is modified ([defns.access]) by adding 1 to it.
The value computation of the ++ expression is sequenced before the modification of the operand object.
With respect to an indeterminately-sequenced function call, the operation of postfix ++ is a single evaluation.
[Note 2:
Therefore, a function call cannot intervene between the lvalue-to-rvalue conversion and the side effect associated with any single postfix ++ operator.
— end note]
The result is a prvalue.
The type of the result is the cv-unqualified version of the type of the operand.
If the operand is a bit-field that cannot represent the incremented value, the resulting value of the bit-field is implementation-defined.
See also [expr.add] and [expr.ass].
2
#
The operand of postfix -- is decremented analogously to the postfix ++ operator.
[Note 3:
For prefix increment and decrement, see [expr.pre.incr].
— end note]

7.6.1.7 Dynamic cast [expr.dynamic.cast]

1
#
The result of the expression dynamic_­cast<T>(v) is the result of converting the expression v to type T.
T shall be a pointer or reference to a complete class type, or “pointer to cv void”.
The dynamic_­cast operator shall not cast away constness ([expr.const.cast]).
2
#
If T is a pointer type, v shall be a prvalue of a pointer to complete class type, and the result is a prvalue of type T.
If T is an lvalue reference type, v shall be an lvalue of a complete class type, and the result is an lvalue of the type referred to by T.
If T is an rvalue reference type, v shall be a glvalue having a complete class type, and the result is an xvalue of the type referred to by T.
3
#
If the type of v is the same as T (ignoring cv-qualifications), the result is v (converted if necessary).
4
#
If T is “pointer to cv1 B” and v has type “pointer to cv2 D” such that B is a base class of D, the result is a pointer to the unique B subobject of the D object pointed to by v, or a null pointer value if v is a null pointer value.
Similarly, if T is “reference to cv1 B” and v has type cv2 D such that B is a base class of D, the result is the unique B subobject of the D object referred to by v.65
In both the pointer and reference cases, the program is ill-formed if B is an inaccessible or ambiguous base class of D.
[Example 1: struct B { }; struct D : B { }; void foo(D* dp) { B* bp = dynamic_cast<B*>(dp); // equivalent to B* bp = dp; } — end example]
5
#
Otherwise, v shall be a pointer to or a glvalue of a polymorphic type.
6
#
If v is a null pointer value, the result is a null pointer value.
7
#
If T is “pointer to cv void”, then the result is a pointer to the most derived object pointed to by v.
Otherwise, a runtime check is applied to see if the object pointed or referred to by v can be converted to the type pointed or referred to by T.
8
#
If C is the class type to which T points or refers, the runtime check logically executes as follows:
  • (8.1)
    If, in the most derived object pointed (referred) to by v, v points (refers) to a public base class subobject of a C object, and if only one object of type C is derived from the subobject pointed (referred) to by v the result points (refers) to that C object.
  • (8.2)
    Otherwise, if v points (refers) to a public base class subobject of the most derived object, and the type of the most derived object has a base class, of type C, that is unambiguous and public, the result points (refers) to the C subobject of the most derived object.
  • (8.3)
    Otherwise, the runtime check fails.
9
#
The value of a failed cast to pointer type is the null pointer value of the required result type.
A failed cast to reference type throws an exception of a type that would match a handler of type std​::​bad_­cast.
[Example 2: class A { virtual void f(); }; class B { virtual void g(); }; class D : public virtual A, private B { }; void g() { D d; B* bp = (B*)&d; // cast needed to break protection A* ap = &d; // public derivation, no cast needed D& dr = dynamic_cast<D&>(*bp); // fails ap = dynamic_cast<A*>(bp); // fails bp = dynamic_cast<B*>(ap); // fails ap = dynamic_cast<A*>(&d); // succeeds bp = dynamic_cast<B*>(&d); // ill-formed (not a runtime check) } class E : public D, public B { }; class F : public E, public D { }; void h() { F f; A* ap = &f; // succeeds: finds unique A D* dp = dynamic_cast<D*>(ap); // fails: yields null; f has two D subobjects E* ep = (E*)ap; // error: cast from virtual base E* ep1 = dynamic_cast<E*>(ap); // succeeds } — end example]
[Note 1:
Subclause [class.cdtor] describes the behavior of a dynamic_­cast applied to an object under construction or destruction.
— end note]
65)
The most derived object ([intro.object]) pointed or referred to by v can contain other B objects as base classes, but these are ignored.
 ⮥

7.6.1.8 Type identification [expr.typeid]

1
#
The result of a typeid expression is an lvalue of static type const std​::​type_­info ([type.info]) and dynamic type const std​::​type_­info or const name where name is an implementation-defined class publicly derived from std​::​type_­info which preserves the behavior described in [type.info].66
The lifetime of the object referred to by the lvalue extends to the end of the program.
Whether or not the destructor is called for the std​::​type_­info object at the end of the program is unspecified.
2
#
When typeid is applied to a glvalue whose type is a polymorphic class type ([class.virtual]), the result refers to a std​::​type_­info object representing the type of the most derived object ([intro.object]) (that is, the dynamic type) to which the glvalue refers.
If the glvalue is obtained by applying the unary * operator to a pointer67 and the pointer is a null pointer value ([basic.compound]), the typeid expression throws an exception ([except.throw]) of a type that would match a handler of type std​::​bad_­typeid exception ([bad.typeid]).
3
#
When typeid is applied to an expression other than a glvalue of a polymorphic class type, the result refers to a std​::​type_­info object representing the static type of the expression.
Lvalue-to-rvalue, array-to-pointer, and function-to-pointer conversions are not applied to the expression.
If the expression is a prvalue, the temporary materialization conversion is applied.
The expression is an unevaluated operand.
4
#
When typeid is applied to a type-id, the result refers to a std​::​type_­info object representing the type of the type-id.
If the type of the type-id is a reference to a possibly cv-qualified type, the result of the typeid expression refers to a std​::​type_­info object representing the cv-unqualified referenced type.
If the type of the type-id is a class type or a reference to a class type, the class shall be completely-defined.
[Note 1:
The type-id cannot denote a function type with a cv-qualifier-seq or a ref-qualifier ([dcl.fct]).
— end note]
5
#
If the type of the expression or type-id is a cv-qualified type, the result of the typeid expression refers to a std​::​type_­info object representing the cv-unqualified type.
[Example 1: class D { /* ... */ }; D d1; const D d2; typeid(d1) == typeid(d2); // yields true typeid(D) == typeid(const D); // yields true typeid(D) == typeid(d2); // yields true typeid(D) == typeid(const D&); // yields true — end example]
6
#
If the header <typeinfo> is not imported or included prior to a use of typeid, the program is ill-formed.
7
#
[Note 2:
Subclause [class.cdtor] describes the behavior of typeid applied to an object under construction or destruction.
— end note]
66)
The recommended name for such a class is extended_­type_­info.
 ⮥
67)
If p is an expression of pointer type, then *p, (*p), *(p), ((*p)), *((p)), and so on all meet this requirement.
 ⮥

7.6.1.9 Static cast [expr.static.cast]

1
#
The result of the expression static_­cast<T>(v) is the result of converting the expression v to type T.
If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue.
The static_­cast operator shall not cast away constness.
2
#
An lvalue of type “cv1 B”, where B is a class type, can be cast to type “reference to cv2 D”, where D is a class derived from B, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.
If B is a virtual base class of D or a base class of a virtual base class of D, or if no valid standard conversion from “pointer to D” to “pointer to B” exists ([conv.ptr]), the program is ill-formed.
An xvalue of type “cv1 B” can be cast to type “rvalue reference to cv2 D” with the same constraints as for an lvalue of type “cv1 B”.
If the object of type “cv1 B” is actually a base class subobject of an object of type D, the result refers to the enclosing object of type D.
Otherwise, the behavior is undefined.
[Example 1: struct B { }; struct D : public B { }; D d; B &br = d; static_cast<D&>(br); // produces lvalue denoting the original d object — end example]
3
#
An lvalue of type T1 can be cast to type “rvalue reference to T2” if T2 is reference-compatible with T1 ([dcl.init.ref]).
If the value is not a bit-field, the result refers to the object or the specified base class subobject thereof; otherwise, the lvalue-to-rvalue conversion is applied to the bit-field and the resulting prvalue is used as the expression of the static_­cast for the remainder of this subclause.
If T2 is an inaccessible or ambiguous base class of T1, a program that necessitates such a cast is ill-formed.
4
#
An expression E can be explicitly converted to a type T if there is an implicit conversion sequence ([over.best.ics]) from E to T, if overload resolution for a direct-initialization ([dcl.init]) of an object or reference of type T from E would find at least one viable function ([over.match.viable]), or if T is an aggregate type ([dcl.init.aggr]) having a first element x and there is an implicit conversion sequence from E to the type of x.
If T is a reference type, the effect is the same as performing the declaration and initialization T t(E); for some invented temporary variable t ([dcl.init]) and then using the temporary variable as the result of the conversion.
Otherwise, the result object is direct-initialized from E.
[Note 1:
The conversion is ill-formed when attempting to convert an expression of class type to an inaccessible or ambiguous base class.
— end note]
[Note 2:
If T is “array of unknown bound of U”, this direct-initialization defines the type of the expression as U[1].
— end note]
5
#
Otherwise, the static_­cast shall perform one of the conversions listed below.
No other conversion shall be performed explicitly using a static_­cast.
6
#
Any expression can be explicitly converted to type cv void, in which case it becomes a discarded-value expression.
[Note 3:
However, if the value is in a temporary object, the destructor for that object is not executed until the usual time, and the value of the object is preserved for the purpose of executing the destructor.
— end note]
7
#
The inverse of any standard conversion sequence not containing an lvalue-to-rvalue, array-to-pointer, function-to-pointer, null pointer, null member pointer, boolean, or function pointer conversion, can be performed explicitly using static_­cast.
A program is ill-formed if it uses static_­cast to perform the inverse of an ill-formed standard conversion sequence.
[Example 2: struct B { }; struct D : private B { }; void f() { static_cast<D*>((B*)0); // error: B is a private base of D static_cast<int B::*>((int D::*)0); // error: B is a private base of D } — end example]
8
#
The lvalue-to-rvalue, array-to-pointer, and function-to-pointer conversions are applied to the operand.
Such a static_­cast is subject to the restriction that the explicit conversion does not cast away constness, and the following additional rules for specific cases:
9
#
A value of a scoped enumeration type ([dcl.enum]) can be explicitly converted to an integral type; the result is the same as that of converting to the enumeration's underlying type and then to the destination type.
A value of a scoped enumeration type can also be explicitly converted to a floating-point type; the result is the same as that of converting from the original value to the floating-point type.
10
#
A value of integral or enumeration type can be explicitly converted to a complete enumeration type.
If the enumeration type has a fixed underlying type, the value is first converted to that type by integral conversion, if necessary, and then to the enumeration type.
If the enumeration type does not have a fixed underlying type, the value is unchanged if the original value is within the range of the enumeration values ([dcl.enum]), and otherwise, the behavior is undefined.
A value of floating-point type can also be explicitly converted to an enumeration type.
The resulting value is the same as converting the original value to the underlying type of the enumeration ([conv.fpint]), and subsequently to the enumeration type.
11
#
A prvalue of type “pointer to cv1 B”, where B is a class type, can be converted to a prvalue of type “pointer to cv2 D”, where D is a complete class derived from B, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.
If B is a virtual base class of D or a base class of a virtual base class of D, or if no valid standard conversion from “pointer to D” to “pointer to B” exists ([conv.ptr]), the program is ill-formed.
The null pointer value ([basic.compound]) is converted to the null pointer value of the destination type.
If the prvalue of type “pointer to cv1 B” points to a B that is actually a subobject of an object of type D, the resulting pointer points to the enclosing object of type D.
Otherwise, the behavior is undefined.
12
#
A prvalue of type “pointer to member of D of type cv1 T” can be converted to a prvalue of type “pointer to member of B of type cv2 T”, where D is a complete class type and B is a base class of D, if cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.
[Note 4:
Function types (including those used in pointer-to-member-function types) are never cv-qualified ([dcl.fct]).
— end note]
If no valid standard conversion from “pointer to member of B of type T” to “pointer to member of D of type T” exists ([conv.mem]), the program is ill-formed.
The null member pointer value is converted to the null member pointer value of the destination type.
If class B contains the original member, or is a base or derived class of the class containing the original member, the resulting pointer to member points to the original member.
Otherwise, the behavior is undefined.
[Note 5:
Although class B need not contain the original member, the dynamic type of the object with which indirection through the pointer to member is performed must contain the original member; see [expr.mptr.oper].
— end note]
13
#
A prvalue of type “pointer to cv1 void” can be converted to a prvalue of type “pointer to cv2 T”, where T is an object type and cv2 is the same cv-qualification as, or greater cv-qualification than, cv1.
If the original pointer value represents the address A of a byte in memory and A does not satisfy the alignment requirement of T, then the resulting pointer value is unspecified.
Otherwise, if the original pointer value points to an object a, and there is an object b of type T (ignoring cv-qualification) that is pointer-interconvertible with a, the result is a pointer to b.
Otherwise, the pointer value is unchanged by the conversion.
[Example 3: T* p1 = new T; const T* p2 = static_cast<const T*>(static_cast<void*>(p1)); bool b = p1 == p2; // b will have the value true. — end example]

7.6.1.10 Reinterpret cast [expr.reinterpret.cast]

1
#
The result of the expression reinterpret_­cast<T>(v) is the result of converting the expression v to type T.
If T is an lvalue reference type or an rvalue reference to function type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the expression v.
Conversions that can be performed explicitly using reinterpret_­cast are listed below.
No other conversion can be performed explicitly using reinterpret_­cast.
2
#
The reinterpret_­cast operator shall not cast away constness.
An expression of integral, enumeration, pointer, or pointer-to-member type can be explicitly converted to its own type; such a cast yields the value of its operand.
3
#
[Note 1:
The mapping performed by reinterpret_­cast might, or might not, produce a representation different from the original value.
— end note]
4
#
A pointer can be explicitly converted to any integral type large enough to hold all values of its type.
The mapping function is implementation-defined.
[Note 2:
It is intended to be unsurprising to those who know the addressing structure of the underlying machine.
— end note]
A value of type std​::​nullptr_­t can be converted to an integral type; the conversion has the same meaning and validity as a conversion of (void*)0 to the integral type.
[Note 3:
A reinterpret_­cast cannot be used to convert a value of any type to the type std​::​nullptr_­t.
— end note]
5
#
A value of integral type or enumeration type can be explicitly converted to a pointer.
A pointer converted to an integer of sufficient size (if any such exists on the implementation) and back to the same pointer type will have its original value; mappings between pointers and integers are otherwise implementation-defined.
[Note 4:
Except as described in [basic.stc.dynamic.safety], the result of such a conversion will not be a safely-derived pointer value.
— end note]
6
#
A function pointer can be explicitly converted to a function pointer of a different type.
[Note 5:
The effect of calling a function through a pointer to a function type ([dcl.fct]) that is not the same as the type used in the definition of the function is undefined ([expr.call]).
— end note]
Except that converting a prvalue of type “pointer to T1” to the type “pointer to T2” (where T1 and T2 are function types) and back to its original type yields the original pointer value, the result of such a pointer conversion is unspecified.
[Note 6:
See also [conv.ptr] for more details of pointer conversions.
— end note]
7
#
An object pointer can be explicitly converted to an object pointer of a different type.68
When a prvalue v of object pointer type is converted to the object pointer type “pointer to cv T”, the result is static_­cast<cv T*>(static_­cast<cv void*>(v)).
[Note 7:
Converting a pointer of type “pointer to T1” that points to an object of type T1 to the type “pointer to T2” (where T2 is an object type and the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer value.
— end note]
8
#
Converting a function pointer to an object pointer type or vice versa is conditionally-supported.
The meaning of such a conversion is implementation-defined, except that if an implementation supports conversions in both directions, converting a prvalue of one type to the other type and back, possibly with different cv-qualification, shall yield the original pointer value.
9
#
The null pointer value ([basic.compound]) is converted to the null pointer value of the destination type.
[Note 8:
A null pointer constant of type std​::​nullptr_­t cannot be converted to a pointer type, and a null pointer constant of integral type is not necessarily converted to a null pointer value.
— end note]
10
#
A prvalue of type “pointer to member of X of type T1” can be explicitly converted to a prvalue of a different type “pointer to member of Y of type T2” if T1 and T2 are both function types or both object types.69
The null member pointer value ([conv.mem]) is converted to the null member pointer value of the destination type.
The result of this conversion is unspecified, except in the following cases:
  • (10.1)
    Converting a prvalue of type “pointer to member function” to a different pointer-to-member-function type and back to its original type yields the original pointer-to-member value.
  • (10.2)
    Converting a prvalue of type “pointer to data member of X of type T1” to the type “pointer to data member of Y of type T2” (where the alignment requirements of T2 are no stricter than those of T1) and back to its original type yields the original pointer-to-member value.
11
#
A glvalue of type T1, designating an object x, can be cast to the type “reference to T2” if an expression of type “pointer to T1” can be explicitly converted to the type “pointer to T2” using a reinterpret_­cast.
The result is that of *reinterpret_­cast<T2 *>(p) where p is a pointer to x of type “pointer to T1”.
No temporary is created, no copy is made, and no constructors ([class.ctor]) or conversion functions ([class.conv]) are called.70
68)
The types can have different cv-qualifiers, subject to the overall restriction that a reinterpret_­cast cannot cast away constness.
 ⮥
69)
T1 and T2 can have different cv-qualifiers, subject to the overall restriction that a reinterpret_­cast cannot cast away constness.
 ⮥
70)
This is sometimes referred to as a type pun when the result refers to the same object as the source glvalue.
 ⮥

7.6.1.11 Const cast [expr.const.cast]

1
#
The result of the expression const_­cast<T>(v) is of type T.
If T is an lvalue reference to object type, the result is an lvalue; if T is an rvalue reference to object type, the result is an xvalue; otherwise, the result is a prvalue and the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the expression v.
Conversions that can be performed explicitly using const_­cast are listed below.
No other conversion shall be performed explicitly using const_­cast.
2
#
[Note 1:
Subject to the restrictions in this subclause, an expression can be cast to its own type using a const_­cast operator.
— end note]
3
#
For two similar types T1 and T2, a prvalue of type T1 may be explicitly converted to the type T2 using a const_­cast if, considering the cv-decompositions of both types, each is the same as for all i.
The result of a const_­cast refers to the original entity.
[Example 1: typedef int *A[3]; // array of 3 pointer to int typedef const int *const CA[3]; // array of 3 const pointer to const int CA &&r = A{}; // OK, reference binds to temporary array object // after qualification conversion to type CA A &&r1 = const_cast<A>(CA{}); // error: temporary array decayed to pointer A &&r2 = const_cast<A&&>(CA{}); // OK — end example]
4
#
For two object types T1 and T2, if a pointer to T1 can be explicitly converted to the type “pointer to T2” using a const_­cast, then the following conversions can also be made:
  • (4.1)
    an lvalue of type T1 can be explicitly converted to an lvalue of type T2 using the cast const_­cast<T2&>;
  • (4.2)
    a glvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_­cast<T2&&>; and
  • (4.3)
    if T1 is a class type, a prvalue of type T1 can be explicitly converted to an xvalue of type T2 using the cast const_­cast<T2&&>.
The result of a reference const_­cast refers to the original object if the operand is a glvalue and to the result of applying the temporary materialization conversion otherwise.
5
#
A null pointer value ([basic.compound]) is converted to the null pointer value of the destination type.
The null member pointer value ([conv.mem]) is converted to the null member pointer value of the destination type.
6
#
[Note 2:
Depending on the type of the object, a write operation through the pointer, lvalue or pointer to data member resulting from a const_­cast that casts away a const-qualifier71 might produce undefined behavior ([dcl.type.cv]).
— end note]
7
#
A conversion from a type T1 to a type T2 casts away constness if T1 and T2 are different, there is a cv-decomposition of T1 yielding n such that T2 has a cv-decomposition of the form ⋯ , and there is no qualification conversion that converts T1 to ⋯ .
8
#
Casting from an lvalue of type T1 to an lvalue of type T2 using an lvalue reference cast or casting from an expression of type T1 to an xvalue of type T2 using an rvalue reference cast casts away constness if a cast from a prvalue of type “pointer to T1” to the type “pointer to T2” casts away constness.
9
#
[Note 3:
Some conversions which involve only changes in cv-qualification cannot be done using const_­cast. For instance, conversions between pointers to functions are not covered because such conversions lead to values whose use causes undefined behavior.
For the same reasons, conversions between pointers to member functions, and in particular, the conversion from a pointer to a const member function to a pointer to a non-const member function, are not covered.
— end note]
71)
const_­cast is not limited to conversions that cast away a const-qualifier.
 ⮥

7.6.2 Unary expressions [expr.unary]

7.6.2.2 Unary operators [expr.unary.op]

1
#
The unary * operator performs indirection: the expression to which it is applied shall be a pointer to an object type, or a pointer to a function type and the result is an lvalue referring to the object or function to which the expression points.
If the type of the expression is “pointer to T”, the type of the result is “T”.
[Note 1:
Indirection through a pointer to an incomplete type (other than cv void) is valid.
The lvalue thus obtained can be used in limited ways (to initialize a reference, for example); this lvalue must not be converted to a prvalue, see [conv.lval].
— end note]
2
#
The result of each of the following unary operators is a prvalue.
3
#
The result of the unary & operator is a pointer to its operand.
  • (3.1)
    If the operand is a qualified-id naming a non-static or variant member m of some class C with type T, the result has type “pointer to member of class C of type T” and is a prvalue designating C​::​m.
  • (3.2)
    Otherwise, if the operand is an lvalue of type T, the resulting expression is a prvalue of type “pointer to T” whose result is a pointer to the designated object ([intro.memory]) or function.
    [Note 2:
    In particular, taking the address of a variable of type “cv T” yields a pointer of type “pointer to cv T”.
    — end note]
  • (3.3)
    Otherwise, the program is ill-formed.
[Example 1: struct A { int i; }; struct B : A { }; ... &B::i ... // has type int A​::​* int a; int* p1 = &a; int* p2 = p1 + 1; // defined behavior bool b = p2 > p1; // defined behavior, with value true — end example]
[Note 3:
A pointer to member formed from a mutable non-static data member ([dcl.stc]) does not reflect the mutable specifier associated with the non-static data member.
— end note]
4
#
A pointer to member is only formed when an explicit & is used and its operand is a qualified-id not enclosed in parentheses.
[Note 4:
That is, the expression &(qualified-id), where the qualified-id is enclosed in parentheses, does not form an expression of type “pointer to member”.
Neither does qualified-id, because there is no implicit conversion from a qualified-id for a non-static member function to the type “pointer to member function” as there is from an lvalue of function type to the type “pointer to function” ([conv.func]).
Nor is &unqualified-id a pointer to member, even within the scope of the unqualified-id's class.
— end note]
5
#
If & is applied to an lvalue of incomplete class type and the complete type declares operator&(), it is unspecified whether the operator has the built-in meaning or the operator function is called.
The operand of & shall not be a bit-field.
6
#
[Note 5:
The address of an overloaded function can be taken only in a context that uniquely determines which version of the overloaded function is referred to (see [over.over]).
Since the context might determine whether the operand is a static or non-static member function, the context can also affect whether the expression has type “pointer to function” or “pointer to member function”.
— end note]
7
#
The operand of the unary + operator shall have arithmetic, unscoped enumeration, or pointer type and the result is the value of the argument.
Integral promotion is performed on integral or enumeration operands.
The type of the result is the type of the promoted operand.
8
#
The operand of the unary - operator shall have arithmetic or unscoped enumeration type and the result is the negative of its operand.
Integral promotion is performed on integral or enumeration operands.
The negative of an unsigned quantity is computed by subtracting its value from , where n is the number of bits in the promoted operand.
The type of the result is the type of the promoted operand.
9
#
The operand of the logical negation operator ! is contextually converted to bool; its value is true if the converted operand is false and false otherwise.
The type of the result is bool.
10
#
The operand of ~ shall have integral or unscoped enumeration type; the result is the ones' complement of its operand.
Integral promotions are performed.
The type of the result is the type of the promoted operand.
There is an ambiguity in the grammar when ~ is followed by a type-name or decltype-specifier.
The ambiguity is resolved by treating ~ as the unary complement operator rather than as the start of an unqualified-id naming a destructor.
[Note 6:
Because the grammar does not permit an operator to follow the ., ->, or ​::​ tokens, a ~ followed by a type-name or decltype-specifier in a member access expression or qualified-id is unambiguously parsed as a destructor name.
— end note]

7.6.2.3 Increment and decrement [expr.pre.incr]

1
#
The operand of prefix ++ is modified ([defns.access]) by adding 1.
The operand shall be a modifiable lvalue.
The type of the operand shall be an arithmetic type other than cv bool, or a pointer to a completely-defined object type.
An operand with volatile-qualified type is deprecated; see [depr.volatile.type].
The result is the updated operand; it is an lvalue, and it is a bit-field if the operand is a bit-field.
The expression ++x is equivalent to x+=1.
[Note 1:
See the discussions of addition and assignment operators for information on conversions.
— end note]
2
#
The operand of prefix -- is modified ([defns.access]) by subtracting 1.
The requirements on the operand of prefix -- and the properties of its result are otherwise the same as those of prefix ++.
[Note 2:
For postfix increment and decrement, see [expr.post.incr].
— end note]

7.6.2.4 Await [expr.await]

1
#
The co_­await expression is used to suspend evaluation of a coroutine ([dcl.fct.def.coroutine]) while awaiting completion of the computation represented by the operand expression.
2
#
An await-expression shall appear only in a potentially-evaluated expression within the compound-statement of a function-body outside of a handler ([except.pre]).
An await-expression shall not appear in a default argument ([dcl.fct.default]).
An await-expression shall not appear in the initializer of a block-scope variable with static or thread storage duration.
A context within a function where an await-expression can appear is called a suspension context of the function.
3
#
Evaluation of an await-expression involves the following auxiliary types, expressions, and objects:
  • (3.1)
    p is an lvalue naming the promise object ([dcl.fct.def.coroutine]) of the enclosing coroutine and P is the type of that object.
  • (3.2)
    a is the cast-expression if the await-expression was implicitly produced by a yield-expression ([expr.yield]), an initial suspend point, or a final suspend point ([dcl.fct.def.coroutine]).
    Otherwise, the unqualified-id await_­transform is looked up within the scope of P by class member access lookup ([basic.lookup.classref]), and if this lookup finds at least one declaration, then a is p.await_­transform(cast-expression); otherwise, a is the cast-expression.
  • (3.3)
    o is determined by enumerating the applicable operator co_­await functions for an argument a ([over.match.oper]), and choosing the best one through overload resolution ([over.match]).
    If overload resolution is ambiguous, the program is ill-formed.
    If no viable functions are found, o is a.
    Otherwise, o is a call to the selected function with the argument a.
    If o would be a prvalue, the temporary materialization conversion ([conv.rval]) is applied.
  • (3.4)
    e is an lvalue referring to the result of evaluating the (possibly-converted) o.
  • (3.5)
    h is an object of type std​::​coroutine_­handle<P> referring to the enclosing coroutine.
  • (3.6)
    await-ready is the expression e.await_­ready(), contextually converted to bool.
  • (3.7)
    await-suspend is the expression e.await_­suspend(h), which shall be a prvalue of type void, bool, or std​::​coroutine_­handle<Z> for some type Z.
  • (3.8)
    await-resume is the expression e.await_­resume().
4
#
The await-expression has the same type and value category as the await-resume expression.
5
#
The await-expression evaluates the (possibly-converted) o expression and the await-ready expression, then:
  • (5.1)
    If the result of await-ready is false, the coroutine is considered suspended.
    Then:
    • (5.1.1)
      If the type of await-suspend is std​::​coroutine_­handle<Z>, await-suspend.resume() is evaluated.
      [Note 1:
      This resumes the coroutine referred to by the result of await-suspend.
      Any number of coroutines can be successively resumed in this fashion, eventually returning control flow to the current coroutine caller or resumer ([dcl.fct.def.coroutine]).
      — end note]
    • (5.1.2)
      Otherwise, if the type of await-suspend is bool, await-suspend is evaluated, and the coroutine is resumed if the result is false.
    • (5.1.3)
      Otherwise, await-suspend is evaluated.
    If the evaluation of await-suspend exits via an exception, the exception is caught, the coroutine is resumed, and the exception is immediately re-thrown ([except.throw]).
    Otherwise, control flow returns to the current coroutine caller or resumer ([dcl.fct.def.coroutine]) without exiting any scopes ([stmt.jump]).
  • (5.2)
    If the result of await-ready is true, or when the coroutine is resumed, the await-resume expression is evaluated, and its result is the result of the await-expression.
6
#
[Example 1: template <typename T> struct my_future { /* ... */ bool await_ready(); void await_suspend(std::coroutine_handle<>); T await_resume(); }; template <class Rep, class Period> auto operator co_await(std::chrono::duration<Rep, Period> d) { struct awaiter { std::chrono::system_clock::duration duration; /* ... */ awaiter(std::chrono::system_clock::duration d) : duration(d) {} bool await_ready() const { return duration.count() <= 0; } void await_resume() {} void await_suspend(std::coroutine_handle<> h) { /* ... */ } }; return awaiter{d}; } using namespace std::chrono; my_future<int> h(); my_future<void> g() { std::cout << "just about go to sleep...\n"; co_await 10ms; std::cout << "resumed\n"; co_await h(); } auto f(int x = co_await h()); // error: await-expression outside of function suspension context int a[] = { co_await h() }; // error: await-expression outside of function suspension context — end example]

7.6.2.5 Sizeof [expr.sizeof]

1
#
The sizeof operator yields the number of bytes occupied by a non-potentially-overlapping object of the type of its operand.
The operand is either an expression, which is an unevaluated operand ([expr.prop]), or a parenthesized type-id.
The sizeof operator shall not be applied to an expression that has function or incomplete type, to the parenthesized name of such types, or to a glvalue that designates a bit-field.
The result of sizeof applied to any of the narrow character types is 1.
The result of sizeof applied to any other fundamental type ([basic.fundamental]) is implementation-defined.
[Note 1:
In particular, the values of sizeof(bool), sizeof(char16_­t), sizeof(char32_­t), and sizeof(wchar_­t) are implementation-defined.72
— end note]
[Note 2:
See [intro.memory] for the definition of byte and [basic.types] for the definition of object representation.
— end note]
2
#
When applied to a reference type, the result is the size of the referenced type.
When applied to a class, the result is the number of bytes in an object of that class including any padding required for placing objects of that type in an array.
The result of applying sizeof to a potentially-overlapping subobject is the size of the type, not the size of the subobject.73
When applied to an array, the result is the total number of bytes in the array.
This implies that the size of an array of n elements is n times the size of an element.
3
#
The lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) standard conversions are not applied to the operand of sizeof.
If the operand is a prvalue, the temporary materialization conversion is applied.
4
#
The identifier in a sizeof... expression shall name a pack.
The sizeof... operator yields the number of elements in the pack ([temp.variadic]).
A sizeof... expression is a pack expansion ([temp.variadic]).
[Example 1: template<class... Types> struct count { static const std::size_t value = sizeof...(Types); }; — end example]
5
#
The result of sizeof and sizeof... is a prvalue of type std​::​size_­t.
[Note 3:
A sizeof expression is an integral constant expression ([expr.const]).
The type std​::​size_­t is defined in the standard header <cstddef> ([cstddef.syn], [support.types.layout]).
— end note]
72)
sizeof(bool) is not required to be 1.
 ⮥
73)
The actual size of a potentially-overlapping subobject can be less than the result of applying sizeof to the subobject, due to virtual base classes and less strict padding requirements on potentially-overlapping subobjects.
 ⮥

7.6.2.6 Alignof [expr.alignof]

1
#
An alignof expression yields the alignment requirement of its operand type.
The operand shall be a type-id representing a complete object type, or an array thereof, or a reference to one of those types.
2
#
The result is a prvalue of type std​::​size_­t.
[Note 1:
An alignof expression is an integral constant expression ([expr.const]).
The type std​::​size_­t is defined in the standard header <cstddef> ([cstddef.syn], [support.types.layout]).
— end note]
3
#
When alignof is applied to a reference type, the result is the alignment of the referenced type.
When alignof is applied to an array type, the result is the alignment of the element type.

7.6.2.7 noexcept operator [expr.unary.noexcept]

1
#
The noexcept operator determines whether the evaluation of its operand, which is an unevaluated operand ([expr.prop]), can throw an exception ([except.throw]).
2
#
The result of the noexcept operator is a prvalue of type bool.
[Note 1:
A noexcept-expression is an integral constant expression ([expr.const]).
— end note]
3
#
The result of the noexcept operator is true unless the expression is potentially-throwing ([except.spec]).

7.6.2.8 New [expr.new]

1
#
The new-expression attempts to create an object of the type-id ([dcl.name]) or new-type-id to which it is applied.
The type of that object is the allocated type.
This type shall be a complete object type, but not an abstract class type or array thereof ([intro.object], [basic.types], [class.abstract]).
[Note 1:
Because references are not objects, references cannot be created by new-expressions.
— end note]
[Note 2:
The type-id can be a cv-qualified type, in which case the object created by the new-expression has a cv-qualified type.
— end note]
2
#
If a placeholder type appears in the type-specifier-seq of a new-type-id or type-id of a new-expression, the allocated type is deduced as follows: Let init be the new-initializer, if any, and T be the new-type-id or type-id of the new-expression, then the allocated type is the type deduced for the variable x in the invented declaration ([dcl.spec.auto]): T x init ;
[Example 1: new auto(1); // allocated type is int auto x = new auto('a'); // allocated type is char, x is of type char* template<class T> struct A { A(T, T); }; auto y = new A{1, 2}; // allocated type is A<int> — end example]
3
#
The new-type-id in a new-expression is the longest possible sequence of new-declarators.
[Note 3:
This prevents ambiguities between the declarator operators &, &&, *, and [] and their expression counterparts.
— end note]
[Example 2: new int * i; // syntax error: parsed as (new int*) i, not as (new int)*i
The * is the pointer declarator and not the multiplication operator.
— end example]
4
#
[Note 4:
Parentheses in a new-type-id of a new-expression can have surprising effects.
[Example 3:
new int(*[10])(); // error is ill-formed because the binding is (new int) (*[10])(); // error
Instead, the explicitly parenthesized version of the new operator can be used to create objects of compound types: new (int (*[10])()); allocates an array of 10 pointers to functions (taking no argument and returning int).
— end example]
— end note]
5
#
Objects created by a new-expression have dynamic storage duration ([basic.stc.dynamic]).
[Note 5:
The lifetime of such an object is not necessarily restricted to the scope in which it is created.
— end note]
When the allocated object is not an array, the result of the new-expression is a pointer to the object created.
6
#
When the allocated object is an array (that is, the noptr-new-declarator syntax is used or the new-type-id or type-id denotes an array type), the new-expression yields a pointer to the initial element (if any) of the array.
[Note 6:
Both new int and new int[10] have type int* and the type of new int[i][10] is int (*)[10]
— end note]
The attribute-specifier-seq in a noptr-new-declarator appertains to the associated array type.
7
#
Every constant-expression in a noptr-new-declarator shall be a converted constant expression ([expr.const]) of type std​::​size_­t and its value shall be greater than zero.
[Example 4:
Given the definition int n = 42, new float[n][5] is well-formed (because n is the expression of a noptr-new-declarator), but new float[5][n] is ill-formed (because n is not a constant expression).
— end example]
8
#
If the type-id or new-type-id denotes an array type of unknown bound ([dcl.array]), the new-initializer shall not be omitted; the allocated object is an array with n elements, where n is determined from the number of initial elements supplied in the new-initializer ([dcl.init.aggr], [dcl.init.string]).
9
#
If the expression in a noptr-new-declarator is present, it is implicitly converted to std​::​size_­t.
The expression is erroneous if:
  • (9.1)
    the expression is of non-class type and its value before converting to std​::​size_­t is less than zero;
  • (9.2)
    the expression is of class type and its value before application of the second standard conversion ([over.ics.user])74 is less than zero;
  • (9.3)
    its value is such that the size of the allocated object would exceed the implementation-defined limit; or
  • (9.4)
    the new-initializer is a braced-init-list and the number of array elements for which initializers are provided (including the terminating '\0' in a string-literal ([lex.string])) exceeds the number of elements to initialize.
If the expression is erroneous after converting to std​::​size_­t:
When the value of the expression is zero, the allocation function is called to allocate an array with no elements.
10
#
A new-expression may obtain storage for the object by calling an allocation function ([basic.stc.dynamic.allocation]).
If the new-expression terminates by throwing an exception, it may release storage by calling a deallocation function.
If the allocated type is a non-array type, the allocation function's name is operator new and the deallocation function's name is operator delete.
If the allocated type is an array type, the allocation function's name is operator new[] and the deallocation function's name is operator delete[].
[Note 7:
An implementation is required to provide default definitions for the global allocation functions ([basic.stc.dynamic], [new.delete.single], [new.delete.array]).
A C++ program can provide alternative definitions of these functions ([replacement.functions]) and/or class-specific versions ([class.free]).
The set of allocation and deallocation functions that can be called by a new-expression could include functions that do not perform allocation or deallocation; for example, see [new.delete.placement].
— end note]
11
#
If the new-expression begins with a unary ​::​ operator, the allocation function's name is looked up in the global scope.
Otherwise, if the allocated type is a class type T or array thereof, the allocation function's name is looked up in the scope of T.
If this lookup fails to find the name, or if the allocated type is not a class type, the allocation function's name is looked up in the global scope.
12
#
An implementation is allowed to omit a call to a replaceable global allocation function ([new.delete.single], [new.delete.array]).
When it does so, the storage is instead provided by the implementation or provided by extending the allocation of another new-expression.
13
#
During an evaluation of a constant expression, a call to an allocation function is always omitted.
[Note 8:
Only new-expressions that would otherwise result in a call to a replaceable global allocation function can be evaluated in constant expressions ([expr.const]).
— end note]
14
#
The implementation may extend the allocation of a new-expression e1 to provide storage for a new-expression e2 if the following would be true were the allocation not extended:
  • (14.1)
    the evaluation of e1 is sequenced before the evaluation of e2, and
  • (14.2)
    e2 is evaluated whenever e1 obtains storage, and
  • (14.3)
    both e1 and e2 invoke the same replaceable global allocation function, and
  • (14.4)
    if the allocation function invoked by e1 and e2 is throwing, any exceptions thrown in the evaluation of either e1 or e2 would be first caught in the same handler, and
  • (14.5)
    the pointer values produced by e1 and e2 are operands to evaluated delete-expressions, and
  • (14.6)
    the evaluation of e2 is sequenced before the evaluation of the delete-expression whose operand is the pointer value produced by e1.
[Example 5: void can_merge(int x) { // These allocations are safe for merging: std::unique_ptr<char[]> a{new (std::nothrow) char[8]}; std::unique_ptr<char[]> b{new (std::nothrow) char[8]}; std::unique_ptr<char[]> c{new (std::nothrow) char[x]}; g(a.get(), b.get(), c.get()); } void cannot_merge(int x) { std::unique_ptr<char[]> a{new char[8]}; try { // Merging this allocation would change its catch handler. std::unique_ptr<char[]> b{new char[x]}; } catch (const std::bad_alloc& e) { std::cerr << "Allocation failed: " << e.what() << std::endl; throw; } } — end example]
15
#
When a new-expression calls an allocation function and that allocation has not been extended, the new-expression passes the amount of space requested to the allocation function as the first argument of type std​::​size_­t.
That argument shall be no less than the size of the object being created; it may be greater than the size of the object being created only if the object is an array and the allocation function is not a non-allocating form ([new.delete.placement]).
For arrays of char, unsigned char, and std​::​byte, the difference between the result of the new-expression and the address returned by the allocation function shall be an integral multiple of the strictest fundamental alignment requirement of any object type whose size is no greater than the size of the array being created.
[Note 9:
Because allocation functions are assumed to return pointers to storage that is appropriately aligned for objects of any type with fundamental alignment, this constraint on array allocation overhead permits the common idiom of allocating character arrays into which objects of other types will later be placed.
— end note]
16
#
When a new-expression calls an allocation function and that allocation has been extended, the size argument to the allocation call shall be no greater than the sum of the sizes for the omitted calls as specified above, plus the size for the extended call had it not been extended, plus any padding necessary to align the allocated objects within the allocated memory.
17
#
The new-placement syntax is used to supply additional arguments to an allocation function; such an expression is called a placement new-expression.
18
#
Overload resolution is performed on a function call created by assembling an argument list.
The first argument is the amount of space requested, and has type std​::​size_­t.
If the type of the allocated object has new-extended alignment, the next argument is the type's alignment, and has type std​::​align_­val_­t.
If the new-placement syntax is used, the initializer-clauses in its expression-list are the succeeding arguments.
If no matching function is found then
  • (18.1)
    if the allocated object type has new-extended alignment, the alignment argument is removed from the argument list;
  • (18.2)
    otherwise, an argument that is the type's alignment and has type std​::​align_­val_­t is added into the argument list immediately after the first argument;
and then overload resolution is performed again.
19
#
[Example 6:
  • (19.1)
    new T results in one of the following calls: operator new(sizeof(T)) operator new(sizeof(T), std::align_val_t(alignof(T)))
  • (19.2)
    new(2,f) T results in one of the following calls: operator new(sizeof(T), 2, f) operator new(sizeof(T), std::align_val_t(alignof(T)), 2, f)
  • (19.3)
    new T[5] results in one of the following calls: operator new[](sizeof(T) * 5 + x) operator new[](sizeof(T) * 5 + x, std::align_val_t(alignof(T)))
  • (19.4)
    new(2,f) T[5] results in one of the following calls: operator new[](sizeof(T) * 5 + x, 2, f) operator new[](sizeof(T) * 5 + x, std::align_val_t(alignof(T)), 2, f)
Here, each instance of x is a non-negative unspecified value representing array allocation overhead; the result of the new-expression will be offset by this amount from the value returned by operator new[].
This overhead may be applied in all array new-expressions, including those referencing a placement allocation function, except when referencing the library function operator new[](std​::​size_­t, void*).
The amount of overhead may vary from one invocation of new to another.
— end example]
20
#
[Note 10:
Unless an allocation function has a non-throwing exception specification, it indicates failure to allocate storage by throwing a std​::​bad_­alloc exception ([basic.stc.dynamic.allocation], [except], [bad.alloc]); it returns a non-null pointer otherwise.
If the allocation function has a non-throwing exception specification, it returns null to indicate failure to allocate storage and a non-null pointer otherwise.
— end note]
If the allocation function is a non-allocating form ([new.delete.placement]) that returns null, the behavior is undefined.
Otherwise, if the allocation function returns null, initialization shall not be done, the deallocation function shall not be called, and the value of the new-expression shall be null.
21
#
[Note 11:
When the allocation function returns a value other than null, it must be a pointer to a block of storage in which space for the object has been reserved.
The block of storage is assumed to be appropriately aligned and of the requested size.
The address of the created object will not necessarily be the same as that of the block if the object is an array.
— end note]
22
#
A new-expression that creates an object of type T initializes that object as follows:
23
#
The invocation of the allocation function is sequenced before the evaluations of expressions in the new-initializer.
Initialization of the allocated object is sequenced before the value computation of the new-expression.
24
#
If the new-expression creates an object or an array of objects of class type, access and ambiguity control are done for the allocation function, the deallocation function ([class.free]), and the constructor ([class.ctor]) selected for the initialization (if any).
If the new-expression creates an array of objects of class type, the destructor is potentially invoked ([class.dtor]).
25
#
If any part of the object initialization described above75 terminates by throwing an exception and a suitable deallocation function can be found, the deallocation function is called to free the memory in which the object was being constructed, after which the exception continues to propagate in the context of the new-expression.
If no unambiguous matching deallocation function can be found, propagating the exception does not cause the object's memory to be freed.
[Note 13:
This is appropriate when the called allocation function does not allocate memory; otherwise, it is likely to result in a memory leak.
— end note]
26
#
If the new-expression begins with a unary ​::​ operator, the deallocation function's name is looked up in the global scope.
Otherwise, if the allocated type is a class type T or an array thereof, the deallocation function's name is looked up in the scope of T.
If this lookup fails to find the name, or if the allocated type is not a class type or array thereof, the deallocation function's name is looked up in the global scope.
27
#
A declaration of a placement deallocation function matches the declaration of a placement allocation function if it has the same number of parameters and, after parameter transformations ([dcl.fct]), all parameter types except the first are identical.
If the lookup finds a single matching deallocation function, that function will be called; otherwise, no deallocation function will be called.
If the lookup finds a usual deallocation function and that function, considered as a placement deallocation function, would have been selected as a match for the allocation function, the program is ill-formed.
For a non-placement allocation function, the normal deallocation function lookup is used to find the matching deallocation function ([expr.delete]).
[Example 7: struct S { // Placement allocation function: static void* operator new(std::size_t, std::size_t); // Usual (non-placement) deallocation function: static void operator delete(void*, std::size_t); }; S* p = new (0) S; // error: non-placement deallocation function matches // placement allocation function — end example]
28
#
If a new-expression calls a deallocation function, it passes the value returned from the allocation function call as the first argument of type void*.
If a placement deallocation function is called, it is passed the same additional arguments as were passed to the placement allocation function, that is, the same arguments as those specified with the new-placement syntax.
If the implementation is allowed to introduce a temporary object or make a copy of any argument as part of the call to the allocation function, it is unspecified whether the same object is used in the call to both the allocation and deallocation functions.
74)
If the conversion function returns a signed integer type, the second standard conversion converts to the unsigned type std​::​size_­t and thus thwarts any attempt to detect a negative value afterwards.
 ⮥
75)
This might include evaluating a new-initializer and/or calling a constructor.
 ⮥

7.6.2.9 Delete [expr.delete]

1
#
The delete-expression operator destroys a most derived object or array created by a new-expression.
delete-expression:
:: delete cast-expression
:: delete [ ] cast-expression
The first alternative is a single-object delete expression, and the second is an array delete expression.
Whenever the delete keyword is immediately followed by empty square brackets, it shall be interpreted as the second alternative.76
The operand shall be of pointer to object type or of class type.
If of class type, the operand is contextually implicitly converted to a pointer to object type.77
The delete-expression's result has type void.
2
#
If the operand has a class type, the operand is converted to a pointer type by calling the above-mentioned conversion function, and the converted operand is used in place of the original operand for the remainder of this subclause.
In a single-object delete expression, the value of the operand of delete may be a null pointer value, a pointer to a non-array object created by a previous new-expression, or a pointer to a subobject representing a base class of such an object.
If not, the behavior is undefined.
In an array delete expression, the value of the operand of delete may be a null pointer value or a pointer value that resulted from a previous array new-expression.78
If not, the behavior is undefined.
[Note 1:
This means that the syntax of the delete-expression must match the type of the object allocated by new, not the syntax of the new-expression.
— end note]
[Note 2:
A pointer to a const type can be the operand of a delete-expression; it is not necessary to cast away the constness of the pointer expression before it is used as the operand of the delete-expression.
— end note]
3
#
In a single-object delete expression, if the static type of the object to be deleted is different from its dynamic type and the selected deallocation function (see below) is not a destroying operator delete, the static type shall be a base class of the dynamic type of the object to be deleted and the static type shall have a virtual destructor or the behavior is undefined.
In an array delete expression, if the dynamic type of the object to be deleted differs from its static type, the behavior is undefined.
4
#
The cast-expression in a delete-expression shall be evaluated exactly once.
5
#
If the object being deleted has incomplete class type at the point of deletion and the complete class has a non-trivial destructor or a deallocation function, the behavior is undefined.
6
#
If the value of the operand of the delete-expression is not a null pointer value and the selected deallocation function (see below) is not a destroying operator delete, the delete-expression will invoke the destructor (if any) for the object or the elements of the array being deleted.
In the case of an array, the elements will be destroyed in order of decreasing address (that is, in reverse order of the completion of their constructor; see [class.base.init]).
7
#
If the value of the operand of the delete-expression is not a null pointer value, then:
[Note 3:
The deallocation function is called regardless of whether the destructor for the object or some element of the array throws an exception.
— end note]
If the value of the operand of the delete-expression is a null pointer value, it is unspecified whether a deallocation function will be called as described above.
8
#
[Note 4:
An implementation provides default definitions of the global deallocation functions operator delete for non-arrays ([new.delete.single]) and operator delete[] for arrays ([new.delete.array]).
A C++ program can provide alternative definitions of these functions ([replacement.functions]), and/or class-specific versions ([class.free]).
— end note]
9
#
When the keyword delete in a delete-expression is preceded by the unary ​::​ operator, the deallocation function's name is looked up in global scope.
Otherwise, the lookup considers class-specific deallocation functions ([class.free]).
If no class-specific deallocation function is found, the deallocation function's name is looked up in global scope.
10
#
If deallocation function lookup finds more than one usual deallocation function, the function to be called is selected as follows:
  • (10.1)
    If any of the deallocation functions is a destroying operator delete, all deallocation functions that are not destroying operator deletes are eliminated from further consideration.
  • (10.2)
    If the type has new-extended alignment, a function with a parameter of type std​::​align_­val_­t is preferred; otherwise a function without such a parameter is preferred.
    If any preferred functions are found, all non-preferred functions are eliminated from further consideration.
  • (10.3)
    If exactly one function remains, that function is selected and the selection process terminates.
  • (10.4)
    If the deallocation functions have class scope, the one without a parameter of type std​::​size_­t is selected.
  • (10.5)
    If the type is complete and if, for an array delete expression only, the operand is a pointer to a class type with a non-trivial destructor or a (possibly multi-dimensional) array thereof, the function with a parameter of type std​::​size_­t is selected.
  • (10.6)
    Otherwise, it is unspecified whether a deallocation function with a parameter of type std​::​size_­t is selected.
11
#
For a single-object delete expression, the deleted object is the object denoted by the operand if its static type does not have a virtual destructor, and its most-derived object otherwise.
[Note 5:
If the deallocation function is not a destroying operator delete and the deleted object is not the most derived object in the former case, the behavior is undefined, as stated above.
— end note]
For an array delete expression, the deleted object is the array object.
When a delete-expression is executed, the selected deallocation function shall be called with the address of the deleted object in a single-object delete expression, or the address of the deleted object suitably adjusted for the array allocation overhead ([expr.new]) in an array delete expression, as its first argument.
[Note 6:
Any cv-qualifiers in the type of the deleted object are ignored when forming this argument.
— end note]
If a destroying operator delete is used, an unspecified value is passed as the argument corresponding to the parameter of type std​::​destroying_­delete_­t.
If a deallocation function with a parameter of type std​::​align_­val_­t is used, the alignment of the type of the deleted object is passed as the corresponding argument.
If a deallocation function with a parameter of type std​::​size_­t is used, the size of the deleted object in a single-object delete expression, or of the array plus allocation overhead in an array delete expression, is passed as the corresponding argument.
[Note 7:
If this results in a call to a replaceable deallocation function, and either the first argument was not the result of a prior call to a replaceable allocation function or the second or third argument was not the corresponding argument in said call, the behavior is undefined ([new.delete.single], [new.delete.array]).
— end note]
12
#
Access and ambiguity control are done for both the deallocation function and the destructor ([class.dtor], [class.free]).
76)
A lambda-expression with a lambda-introducer that consists of empty square brackets can follow the delete keyword if the lambda-expression is enclosed in parentheses.
 ⮥
77)
This implies that an object cannot be deleted using a pointer of type void* because void is not an object type.
 ⮥
78)
For nonzero-length arrays, this is the same as a pointer to the first element of the array created by that new-expression.
Zero-length arrays do not have a first element.
 ⮥

7.6.3 Explicit type conversion (cast notation) [expr.cast]

1
#
The result of the expression (T) cast-expression is of type T.
The result is an lvalue if T is an lvalue reference type or an rvalue reference to function type and an xvalue if T is an rvalue reference to object type; otherwise the result is a prvalue.
[Note 1:
If T is a non-class type that is cv-qualified, the cv-qualifiers are discarded when determining the type of the resulting prvalue; see [expr.prop].
— end note]
2
#
An explicit type conversion can be expressed using functional notation, a type conversion operator (dynamic_­cast, static_­cast, reinterpret_­cast, const_­cast), or the cast notation.
3
#
Any type conversion not mentioned below and not explicitly defined by the user ([class.conv]) is ill-formed.
4
#
The conversions performed by can be performed using the cast notation of explicit type conversion.
The same semantic restrictions and behaviors apply, with the exception that in performing a static_­cast in the following situations the conversion is valid even if the base class is inaccessible:
  • (4.6)
    a pointer to an object of derived class type or an lvalue or rvalue of derived class type may be explicitly converted to a pointer or reference to an unambiguous base class type, respectively;
  • (4.7)
    a pointer to member of derived class type may be explicitly converted to a pointer to member of an unambiguous non-virtual base class type;
  • (4.8)
    a pointer to an object of an unambiguous non-virtual base class type, a glvalue of an unambiguous non-virtual base class type, or a pointer to member of an unambiguous non-virtual base class type may be explicitly converted to a pointer, a reference, or a pointer to member of a derived class type, respectively.
If a conversion can be interpreted in more than one of the ways listed above, the interpretation that appears first in the list is used, even if a cast resulting from that interpretation is ill-formed.
If a conversion can be interpreted in more than one way as a static_­cast followed by a const_­cast, the conversion is ill-formed.
[Example 1: struct A { }; struct I1 : A { }; struct I2 : A { }; struct D : I1, I2 { }; A* foo( D* p ) { return (A*)( p ); // ill-formed static_­cast interpretation } — end example]
5
#
The operand of a cast using the cast notation can be a prvalue of type “pointer to incomplete class type”.
The destination type of a cast using the cast notation can be “pointer to incomplete class type”.
If both the operand and destination types are class types and one or both are incomplete, it is unspecified whether the static_­cast or the reinterpret_­cast interpretation is used, even if there is an inheritance relationship between the two classes.
[Note 2:
For example, if the classes were defined later in the translation unit, a multi-pass compiler would be permitted to interpret a cast between pointers to the classes as if the class types were complete at the point of the cast.
— end note]

7.6.4 Pointer-to-member operators [expr.mptr.oper]

1
#
The pointer-to-member operators ->* and .* group left-to-right.
2
#
The binary operator .* binds its second operand, which shall be of type “pointer to member of T” to its first operand, which shall be a glvalue of class T or of a class of which T is an unambiguous and accessible base class.
The result is an object or a function of the type specified by the second operand.
3
#
The binary operator ->* binds its second operand, which shall be of type “pointer to member of T” to its first operand, which shall be of type “pointer to U” where U is either T or a class of which T is an unambiguous and accessible base class.
The expression E1->*E2 is converted into the equivalent form (*(E1)).*E2.
4
#
Abbreviating pm-expression.*cast-expression as E1.*E2, E1 is called the object expression.
If the dynamic type of E1 does not contain the member to which E2 refers, the behavior is undefined.
Otherwise, the expression E1 is sequenced before the expression E2.
5
#
The restrictions on cv-qualification, and the manner in which the cv-qualifiers of the operands are combined to produce the cv-qualifiers of the result, are the same as the rules for E1.E2 given in [expr.ref].
[Note 1:
It is not possible to use a pointer to member that refers to a mutable member to modify a const class object.
For example, struct S { S() : i(0) { } mutable int i; }; void f() { const S cs; int S::* pm = &S::i; // pm refers to mutable member S​::​i cs.*pm = 88; // error: cs is a const object }
— end note]
6
#
If the result of .* or ->* is a function, then that result can be used only as the operand for the function call operator ().
[Example 1:
(ptr_to_obj->*ptr_to_mfct)(10); calls the member function denoted by ptr_­to_­mfct for the object pointed to by ptr_­to_­obj.
— end example]
In a .* expression whose object expression is an rvalue, the program is ill-formed if the second operand is a pointer to member function whose ref-qualifier is &, unless its cv-qualifier-seq is const.
In a .* expression whose object expression is an lvalue, the program is ill-formed if the second operand is a pointer to member function whose ref-qualifier is &&.
The result of a .* expression whose second operand is a pointer to a data member is an lvalue if the first operand is an lvalue and an xvalue otherwise.
The result of a .* expression whose second operand is a pointer to a member function is a prvalue.
If the second operand is the null member pointer value, the behavior is undefined.

7.6.5 Multiplicative operators [expr.mul]

2
#
The operands of * and / shall have arithmetic or unscoped enumeration type; the operands of % shall have integral or unscoped enumeration type.
The usual arithmetic conversions are performed on the operands and determine the type of the result.
3
#
The binary * operator indicates multiplication.
4
#
The binary / operator yields the quotient, and the binary % operator yields the remainder from the division of the first expression by the second.
If the second operand of / or % is zero the behavior is undefined.
For integral operands the / operator yields the algebraic quotient with any fractional part discarded;79 if the quotient a/b is representable in the type of the result, (a/b)*b + a%b is equal to a; otherwise, the behavior of both a/b and a%b is undefined.
79)
This is often called truncation towards zero.
 ⮥

7.6.6 Additive operators [expr.add]

1
#
The additive operators + and - group left-to-right.
The usual arithmetic conversions are performed for operands of arithmetic or enumeration type.
For addition, either both operands shall have arithmetic or unscoped enumeration type, or one operand shall be a pointer to a completely-defined object type and the other shall have integral or unscoped enumeration type.
2
#
For subtraction, one of the following shall hold:
  • (2.1)
    both operands have arithmetic or unscoped enumeration type; or
  • (2.2)
    both operands are pointers to cv-qualified or cv-unqualified versions of the same completely-defined object type; or
  • (2.3)
    the left operand is a pointer to a completely-defined object type and the right operand has integral or unscoped enumeration type.
3
#
The result of the binary + operator is the sum of the operands.
The result of the binary - operator is the difference resulting from the subtraction of the second operand from the first.
4
#
When an expression J that has integral type is added to or subtracted from an expression P of pointer type, the result has the type of P.
  • (4.1)
    If P evaluates to a null pointer value and J evaluates to 0, the result is a null pointer value.
  • (4.2)
    Otherwise, if P points to an array element i of an array object x with n elements ([dcl.array]),80 the expressions P + J and J + P (where J has the value j) point to the (possibly-hypothetical) array element of x if and the expression P - J points to the (possibly-hypothetical) array element of x if .
  • (4.3)
    Otherwise, the behavior is undefined.
5
#
When two pointer expressions P and Q are subtracted, the type of the result is an implementation-defined signed integral type; this type shall be the same type that is defined as std​::​ptrdiff_­t in the <cstddef> header ([support.types.layout]).
  • (5.1)
    If P and Q both evaluate to null pointer values, the result is 0.
  • (5.2)
    Otherwise, if P and Q point to, respectively, array elements i and j of the same array object x, the expression P - Q has the value .
  • (5.3)
    Otherwise, the behavior is undefined.
    [Note 1:
    If the value is not in the range of representable values of type std​::​ptrdiff_­t, the behavior is undefined.
    — end note]
6
#
For addition or subtraction, if the expressions P or Q have type “pointer to cv T”, where T and the array element type are not similar, the behavior is undefined.
[Note 2:
In particular, a pointer to a base class cannot be used for pointer arithmetic when the array contains objects of a derived class type.
— end note]
80)
As specified in [basic.compound], an object that is not an array element is considered to belong to a single-element array for this purpose and a pointer past the last element of an array of n elements is considered to be equivalent to a pointer to a hypothetical array element n for this purpose.
 ⮥

7.6.7 Shift operators [expr.shift]

1
#
The shift operators << and >> group left-to-right.
The operands shall be of integral or unscoped enumeration type and integral promotions are performed.
The type of the result is that of the promoted left operand.
The behavior is undefined if the right operand is negative, or greater than or equal to the width of the promoted left operand.
2
#
The value of E1 << E2 is the unique value congruent to modulo , where N is the width of the type of the result.
[Note 1:
E1 is left-shifted E2 bit positions; vacated bits are zero-filled.
— end note]
3
#
The value of E1 >> E2 is , rounded down.
[Note 2:
E1 is right-shifted E2 bit positions.
Right-shift on signed integral types is an arithmetic right shift, which performs sign-extension.
— end note]
4
#
The expression E1 is sequenced before the expression E2.

7.6.8 Three-way comparison operator [expr.spaceship]

1
#
The three-way comparison operator groups left-to-right.
2
#
The expression p <=> q is a prvalue indicating whether p is less than, equal to, greater than, or incomparable with q.
3
#
If one of the operands is of type bool and the other is not, the program is ill-formed.
4
#
If both operands have arithmetic types, or one operand has integral type and the other operand has unscoped enumeration type, the usual arithmetic conversions are applied to the operands.
Then:
  • (4.1)
    If a narrowing conversion is required, other than from an integral type to a floating-point type, the program is ill-formed.
  • (4.2)
    Otherwise, if the operands have integral type, the result is of type std​::​strong_­ordering.
    The result is std​::​strong_­ordering​::​equal if both operands are arithmetically equal, std​::​strong_­ordering​::​less if the first operand is arithmetically less than the second operand, and std​::​strong_­ordering​::​greater otherwise.
  • (4.3)
    Otherwise, the operands have floating-point type, and the result is of type std​::​partial_­ordering.
    The expression a <=> b yields std​::​partial_­ordering​::​less if a is less than b, std​::​partial_­ordering​::​greater if a is greater than b, std​::​partial_­ordering​::​equivalent if a is equivalent to b, and std​::​partial_­ordering​::​unordered otherwise.
5
#
If both operands have the same enumeration type E, the operator yields the result of converting the operands to the underlying type of E and applying <=> to the converted operands.
6
#
If at least one of the operands is of object pointer type and the other operand is of object pointer or array type, array-to-pointer conversions ([conv.array]), pointer conversions ([conv.ptr]), and qualification conversions are performed on both operands to bring them to their composite pointer type ([expr.type]).
After the conversions, the operands shall have the same type.
[Note 1:
If both of the operands are arrays, array-to-pointer conversions are not applied.
— end note]
In this case, p <=> q is of type std​::​strong_­ordering and the result is defined by the following rules:
  • (6.1)
    If two pointer operands p and q compare equal ([expr.eq]), p <=> q yields std​::​strong_­ordering​::​equal;
  • (6.2)
    otherwise, if p and q compare unequal, p <=> q yields std​::​strong_­ordering​::​less if q compares greater than p and std​::​strong_­ordering​::​greater if p compares greater than q ([expr.rel]);
  • (6.3)
    otherwise, the result is unspecified.
7
#
Otherwise, the program is ill-formed.
8
#
The three comparison category types ([cmp.categories]) (the types std​::​strong_­ordering, std​::​weak_­ordering, and std​::​partial_­ordering) are not predefined; if the header <compare> is not imported or included prior to a use of such a class type – even an implicit use in which the type is not named (e.g., via the auto specifier in a defaulted three-way comparison or use of the built-in operator) – the program is ill-formed.

7.6.9 Relational operators [expr.rel]

1
#
The relational operators group left-to-right.
[Example 1:
a<b<c means (a<b)<c and not (a<b)&&(b<c).
— end example]
The comparison is deprecated if both operands were of array type prior to these conversions ([depr.array.comp]).
2
#
The converted operands shall have arithmetic, enumeration, or pointer type.
The operators < (less than), > (greater than), <= (less than or equal to), and >= (greater than or equal to) all yield false or true.
The type of the result is bool.
3
#
The usual arithmetic conversions are performed on operands of arithmetic or enumeration type.
If both operands are pointers, pointer conversions and qualification conversions are performed to bring them to their composite pointer type.
After conversions, the operands shall have the same type.
4
#
The result of comparing unequal pointers to objects81 is defined in terms of a partial order consistent with the following rules:
  • (4.1)
    If two pointers point to different elements of the same array, or to subobjects thereof, the pointer to the element with the higher subscript is required to compare greater.
  • (4.2)
    If two pointers point to different non-static data members of the same object, or to subobjects of such members, recursively, the pointer to the later declared member is required to compare greater provided the two members have the same access control ([class.access]), neither member is a subobject of zero size, and their class is not a union.
  • (4.3)
    Otherwise, neither pointer is required to compare greater than the other.
5
#
If two operands p and q compare equal, p<=q and p>=q both yield true and p<q and p>q both yield false.
Otherwise, if a pointer p compares greater than a pointer q, p>=q, p>q, q<=p, and q<p all yield true and p<=q, p<q, q>=p, and q>p all yield false.
Otherwise, the result of each of the operators is unspecified.
6
#
If both operands (after conversions) are of arithmetic or enumeration type, each of the operators shall yield true if the specified relationship is true and false if it is false.
81)
As specified in [basic.compound], an object that is not an array element is considered to belong to a single-element array for this purpose and a pointer past the last element of an array of n elements is considered to be equivalent to a pointer to a hypothetical array element n for this purpose.
 ⮥

7.6.10 Equality operators [expr.eq]

1
#
The == (equal to) and the != (not equal to) operators group left-to-right.
The lvalue-to-rvalue ([conv.lval]), array-to-pointer ([conv.array]), and function-to-pointer ([conv.func]) standard conversions are performed on the operands.
The comparison is deprecated if both operands were of array type prior to these conversions ([depr.array.comp]).
2
#
The converted operands shall have arithmetic, enumeration, pointer, or pointer-to-member type, or type std​::​nullptr_­t.
The operators == and != both yield true or false, i.e., a result of type bool.
In each case below, the operands shall have the same type after the specified conversions have been applied.
3
#
If at least one of the operands is a pointer, pointer conversions, function pointer conversions, and qualification conversions are performed on both operands to bring them to their composite pointer type.
Comparing pointers is defined as follows:
  • (3.1)
    If one pointer represents the address of a complete object, and another pointer represents the address one past the last element of a different complete object,82 the result of the comparison is unspecified.
  • (3.2)
    Otherwise, if the pointers are both null, both point to the same function, or both represent the same address, they compare equal.
  • (3.3)
    Otherwise, the pointers compare unequal.
4
#
If at least one of the operands is a pointer to member, pointer-to-member conversions ([conv.mem]), function pointer conversions ([conv.fctptr]), and qualification conversions ([conv.qual]) are performed on both operands to bring them to their composite pointer type ([expr.type]).
Comparing pointers to members is defined as follows:
  • (4.1)
    If two pointers to members are both the null member pointer value, they compare equal.
  • (4.2)
    If only one of two pointers to members is the null member pointer value, they compare unequal.
  • (4.3)
    If either is a pointer to a virtual member function, the result is unspecified.
  • (4.4)
    If one refers to a member of class C1 and the other refers to a member of a different class C2, where neither is a base class of the other, the result is unspecified.
    [Example 1: struct A {}; struct B : A { int x; }; struct C : A { int x; }; int A::*bx = (int(A::*))&B::x; int A::*cx = (int(A::*))&C::x; bool b1 = (bx == cx); // unspecified — end example]
  • (4.5)
    If both refer to (possibly different) members of the same union, they compare equal.
  • (4.6)
    Otherwise, two pointers to members compare equal if they would refer to the same member of the same most derived object or the same subobject if indirection with a hypothetical object of the associated class type were performed, otherwise they compare unequal.
    [Example 2: struct B { int f(); }; struct L : B { }; struct R : B { }; struct D : L, R { }; int (B::*pb)() = &B::f; int (L::*pl)() = pb; int (R::*pr)() = pb; int (D::*pdl)() = pl; int (D::*pdr)() = pr; bool x = (pdl == pdr); // false bool y = (pb == pl); // true — end example]
5
#
Two operands of type std​::​nullptr_­t or one operand of type std​::​nullptr_­t and the other a null pointer constant compare equal.
6
#
If two operands compare equal, the result is true for the == operator and false for the != operator.
If two operands compare unequal, the result is false for the == operator and true for the != operator.
Otherwise, the result of each of the operators is unspecified.
7
#
If both operands are of arithmetic or enumeration type, the usual arithmetic conversions are performed on both operands; each of the operators shall yield true if the specified relationship is true and false if it is false.
82)
As specified in [basic.compound], an object that is not an array element is considered to belong to a single-element array for this purpose.
 ⮥

7.6.11 Bitwise AND operator [expr.bit.and]

1
#
The & operator groups left-to-right.
The operands shall be of integral or unscoped enumeration type.
The usual arithmetic conversions ([expr.arith.conv]) are performed.
Given the coefficients and of the base-2 representation ([basic.fundamental]) of the converted operands x and y, the coefficient of the base-2 representation of the result r is 1 if both and are 1, and 0 otherwise.
[Note 1:
The result is the bitwise AND function of the operands.
— end note]

7.6.12 Bitwise exclusive OR operator [expr.xor]

1
#
The ^ operator groups left-to-right.
The operands shall be of integral or unscoped enumeration type.
The usual arithmetic conversions ([expr.arith.conv]) are performed.
Given the coefficients and of the base-2 representation ([basic.fundamental]) of the converted operands x and y, the coefficient of the base-2 representation of the result r is 1 if either (but not both) of and are 1, and 0 otherwise.
[Note 1:
The result is the bitwise exclusive OR function of the operands.
— end note]

7.6.13 Bitwise inclusive OR operator [expr.or]

1
#
The | operator groups left-to-right.
The operands shall be of integral or unscoped enumeration type.
The usual arithmetic conversions ([expr.arith.conv]) are performed.
Given the coefficients and of the base-2 representation ([basic.fundamental]) of the converted operands x and y, the coefficient of the base-2 representation of the result r is 1 if at least one of and are 1, and 0 otherwise.
[Note 1:
The result is the bitwise inclusive OR function of the operands.
— end note]

7.6.14 Logical AND operator [expr.log.and]

1
#
The && operator groups left-to-right.
The operands are both contextually converted to bool.
The result is true if both operands are true and false otherwise.
Unlike &, && guarantees left-to-right evaluation: the second operand is not evaluated if the first operand is false.
2
#
The result is a bool.
If the second expression is evaluated, the first expression is sequenced before the second expression ([intro.execution]).

7.6.15 Logical OR operator [expr.log.or]

1
#
The || operator groups left-to-right.
The operands are both contextually converted to bool.
The result is true if either of its operands is true, and false otherwise.
Unlike |, || guarantees left-to-right evaluation; moreover, the second operand is not evaluated if the first operand evaluates to true.
2
#
The result is a bool.
If the second expression is evaluated, the first expression is sequenced before the second expression ([intro.execution]).

7.6.16 Conditional operator [expr.cond]

1
#
Conditional expressions group right-to-left.
The first expression is contextually converted to bool.
It is evaluated and if it is true, the result of the conditional expression is the value of the second expression, otherwise that of the third expression.
Only one of the second and third expressions is evaluated.
The first expression is sequenced before the second or third expression ([intro.execution]).
2
#
If either the second or the third operand has type void, one of the following shall hold:
  • (2.1)
    The second or the third operand (but not both) is a (possibly parenthesized) throw-expression ([expr.throw]); the result is of the type and value category of the other.
    The conditional-expression is a bit-field if that operand is a bit-field.
  • (2.2)
    Both the second and the third operands have type void; the result is of type void and is a prvalue.
    [Note 1:
    This includes the case where both operands are throw-expressions.
    — end note]
3
#
Otherwise, if the second and third operand are glvalue bit-fields of the same value category and of types cv1 T and cv2 T, respectively, the operands are considered to be of type cv T for the remainder of this subclause, where cv is the union of cv1 and cv2.
4
#
Otherwise, if the second and third operand have different types and either has (possibly cv-qualified) class type, or if both are glvalues of the same value category and the same type except for cv-qualification, an attempt is made to form an implicit conversion sequence from each of those operands to the type of the other.
[Note 2:
Properties such as access, whether an operand is a bit-field, or whether a conversion function is deleted are ignored for that determination.
— end note]
Attempts are made to form an implicit conversion sequence from an operand expression E1 of type T1 to a target type related to the type T2 of the operand expression E2 as follows:
  • (4.1)
    If E2 is an lvalue, the target type is “lvalue reference to T2”, but an implicit conversion sequence can only be formed if the reference would bind directly ([dcl.init.ref]) to a glvalue.
  • (4.2)
    If E2 is an xvalue, the target type is “rvalue reference to T2”, but an implicit conversion sequence can only be formed if the reference would bind directly.
  • (4.3)
    If E2 is a prvalue or if neither of the conversion sequences above can be formed and at least one of the operands has (possibly cv-qualified) class type:
    • (4.3.1)
      if T1 and T2 are the same class type (ignoring cv-qualification) and T2 is at least as cv-qualified as T1, the target type is T2,
    • (4.3.2)
      otherwise, if T2 is a base class of T1, the target type is cv1 T2, where cv1 denotes the cv-qualifiers of T1,
    • (4.3.3)
      otherwise, the target type is the type that E2 would have after applying the lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions.
Using this process, it is determined whether an implicit conversion sequence can be formed from the second operand to the target type determined for the third operand, and vice versa.
If both sequences can be formed, or one can be formed but it is the ambiguous conversion sequence, the program is ill-formed.
If no conversion sequence can be formed, the operands are left unchanged and further checking is performed as described below.
Otherwise, if exactly one conversion sequence can be formed, that conversion is applied to the chosen operand and the converted operand is used in place of the original operand for the remainder of this subclause.
[Note 3:
The conversion might be ill-formed even if an implicit conversion sequence could be formed.
— end note]
5
#
If the second and third operands are glvalues of the same value category and have the same type, the result is of that type and value category and it is a bit-field if the second or the third operand is a bit-field, or if both are bit-fields.
6
#
Otherwise, the result is a prvalue.
If the second and third operands do not have the same type, and either has (possibly cv-qualified) class type, overload resolution is used to determine the conversions (if any) to be applied to the operands ([over.match.oper], [over.built]).
If the overload resolution fails, the program is ill-formed.
Otherwise, the conversions thus determined are applied, and the converted operands are used in place of the original operands for the remainder of this subclause.
7
#
Lvalue-to-rvalue, array-to-pointer, and function-to-pointer standard conversions are performed on the second and third operands.
After those conversions, one of the following shall hold:
  • (7.1)
    The second and third operands have the same type; the result is of that type and the result object is initialized using the selected operand.
  • (7.2)
    The second and third operands have arithmetic or enumeration type; the usual arithmetic conversions are performed to bring them to a common type, and the result is of that type.
  • (7.3)
    One or both of the second and third operands have pointer type; pointer conversions, function pointer conversions, and qualification conversions are performed to bring them to their composite pointer type.
    The result is of the composite pointer type.
  • (7.4)
    One or both of the second and third operands have pointer-to-member type; pointer to member conversions ([conv.mem]), function pointer conversions ([conv.fctptr]), and qualification conversions ([conv.qual]) are performed to bring them to their composite pointer type ([expr.type]).
    The result is of the composite pointer type.
  • (7.5)
    Both the second and third operands have type std​::​nullptr_­t or one has that type and the other is a null pointer constant.
    The result is of type std​::​nullptr_­t.

7.6.17 Yielding a value [expr.yield]

1
#
A yield-expression shall appear only within a suspension context of a function ([expr.await]).
Let e be the operand of the yield-expression and p be an lvalue naming the promise object of the enclosing coroutine ([dcl.fct.def.coroutine]), then the yield-expression is equivalent to the expression co_­await p.yield_­value(e).
[Example 1: template <typename T> struct my_generator { struct promise_type { T current_value; /* ... */ auto yield_value(T v) { current_value = std::move(v); return std::suspend_always{}; } }; struct iterator { /* ... */ }; iterator begin(); iterator end(); }; my_generator<pair<int,int>> g1() { for (int i = i; i < 10; ++i) co_yield {i,i}; } my_generator<pair<int,int>> g2() { for (int i = i; i < 10; ++i) co_yield make_pair(i,i); } auto f(int x = co_yield 5); // error: yield-expression outside of function suspension context int a[] = { co_yield 1 }; // error: yield-expression outside of function suspension context int main() { auto r1 = g1(); auto r2 = g2(); assert(std::equal(r1.begin(), r1.end(), r2.begin(), r2.end())); } — end example]

7.6.18 Throwing an exception [expr.throw]

1
#
A throw-expression is of type void.
2
#
Evaluating a throw-expression with an operand throws an exception; the type of the exception object is determined by removing any top-level cv-qualifiers from the static type of the operand and adjusting the type from “array of T” or function type T to “pointer to T”.
3
#
A throw-expression with no operand rethrows the currently handled exception.
The exception is reactivated with the existing exception object; no new exception object is created.
The exception is no longer considered to be caught.
[Example 1:
An exception handler that cannot completely handle the exception itself can be written like this: try { // ... } catch (...) { // catch all exceptions // respond (partially) to exception throw; // pass the exception to some other handler }
— end example]
4
#
If no exception is presently being handled, evaluating a throw-expression with no operand calls std​::​​terminate().

7.6.19 Assignment and compound assignment operators [expr.ass]

1
#
The assignment operator (=) and the compound assignment operators all group right-to-left.
All require a modifiable lvalue as their left operand; their result is an lvalue referring to the left operand.
The result in all cases is a bit-field if the left operand is a bit-field.
In all cases, the assignment is sequenced after the value computation of the right and left operands, and before the value computation of the assignment expression.
The right operand is sequenced before the left operand.
With respect to an indeterminately-sequenced function call, the operation of a compound assignment is a single evaluation.
[Note 1:
Therefore, a function call cannot intervene between the lvalue-to-rvalue conversion and the side effect associated with any single compound assignment operator.
— end note]
assignment-operator: one of
= *= /= %= += -= >>= <<= &= ^= |=
2
#
In simple assignment (=), the object referred to by the left operand is modified ([defns.access]) by replacing its value with the result of the right operand.
3
#
If the right operand is an expression, it is implicitly converted to the cv-unqualified type of the left operand.
4
#
When the left operand of an assignment operator is a bit-field that cannot represent the value of the expression, the resulting value of the bit-field is implementation-defined.
5
#
A simple assignment whose left operand is of a volatile-qualified type is deprecated ([depr.volatile.type]) unless the (possibly parenthesized) assignment is a discarded-value expression or an unevaluated operand.
6
#
The behavior of an expression of the form E1 op= E2 is equivalent to E1 = E1 op E2 except that E1 is evaluated only once.
Such expressions are deprecated if E1 has volatile-qualified type; see [depr.volatile.type].
For += and -=, E1 shall either have arithmetic type or be a pointer to a possibly cv-qualified completely-defined object type.
In all other cases, E1 shall have arithmetic type.
7
#
If the value being stored in an object is read via another object that overlaps in any way the storage of the first object, then the overlap shall be exact and the two objects shall have the same type, otherwise the behavior is undefined.
[Note 2:
This restriction applies to the relationship between the left and right sides of the assignment operation; it is not a statement about how the target of the assignment might be aliased in general.
— end note]
8
#
A braced-init-list may appear on the right-hand side of
  • (8.1)
    an assignment to a scalar, in which case the initializer list shall have at most a single element.
    The meaning of x = {v}, where T is the scalar type of the expression x, is that of x = T{v}.
    The meaning of x = {} is x = T{}.
  • (8.2)
    an assignment to an object of class type, in which case the initializer list is passed as the argument to the assignment operator function selected by overload resolution ([over.ass], [over.match]).
[Example 1: complex<double> z; z = { 1,2 }; // meaning z.operator=({1,2}) z += { 1, 2 }; // meaning z.operator+=({1,2}) int a, b; a = b = { 1 }; // meaning a=b=1; a = { 1 } = b; // syntax error — end example]

7.6.20 Comma operator [expr.comma]

1
#
The comma operator groups left-to-right.
A pair of expressions separated by a comma is evaluated left-to-right; the left expression is a discarded-value expression.
The left expression is sequenced before the right expression ([intro.execution]).
The type and value of the result are the type and value of the right operand; the result is of the same value category as its right operand, and is a bit-field if its right operand is a bit-field.
2
#
[Note 1:
In contexts where the comma token is given special meaning (e.g. function calls ([expr.call]), lists of initializers ([dcl.init]), or template-argument-lists ([temp.names])), the comma operator as described in this subclause can appear only in parentheses.
[Example 1:
f(a, (t=3, t+2), c); has three arguments, the second of which has the value 5.
— end example]
— end note]
3
#
[Note 2:
A comma expression appearing as the expr-or-braced-init-list of a subscripting expression is deprecated; see [depr.comma.subscript].
— end note]

7.7 Constant expressions [expr.const]

1
#
Certain contexts require expressions that satisfy additional requirements as detailed in this subclause; other contexts have different semantics depending on whether or not an expression satisfies these requirements.
Expressions that satisfy these requirements, assuming that copy elision is not performed, are called constant expressions.
[Note 1:
Constant expressions can be evaluated during translation.
— end note]
2
#
A variable or temporary object o is constant-initialized if
  • (2.1)
    either it has an initializer or its default-initialization results in some initialization being performed, and
  • (2.2)
    the full-expression of its initialization is a constant expression when interpreted as a constant-expression, except that if o is an object, that full-expression may also invoke constexpr constructors for o and its subobjects even if those objects are of non-literal class types.
    [Note 2:
    Such a class can have a non-trivial destructor.
    Within this evaluation, std​::​is_­constant_­evaluated() ([meta.const.eval]) returns true.
    — end note]
3
#
A variable is potentially-constant if it is constexpr or it has reference or const-qualified integral or enumeration type.
4
#
A constant-initialized potentially-constant variable V is usable in constant expressions at a point P if V's initializing declaration D is reachable from P and
  • (4.1)
    V is constexpr,
  • (4.2)
    V is not initialized to a TU-local value, or
  • (4.3)
    P is in the same translation unit as D.
An object or reference is usable in constant expressions if it is
  • (4.4)
    a variable that is usable in constant expressions, or
  • (4.5)
    a template parameter object, or
  • (4.6)
    a string literal object, or
  • (4.7)
    a temporary object of non-volatile const-qualified literal type whose lifetime is extended ([class.temporary]) to that of a variable that is usable in constant expressions, or
  • (4.8)
    a non-mutable subobject or reference member of any of the above.
5
#
An expression E is a core constant expression unless the evaluation of E, following the rules of the abstract machine ([intro.execution]), would evaluate one of the following:
  • (5.1)
    this, except in a constexpr function that is being evaluated as part of E;
  • (5.2)
    an invocation of a non-constexpr function83;
  • (5.3)
    an invocation of an undefined constexpr function;
  • (5.4)
    an invocation of an instantiated constexpr function that fails to satisfy the requirements for a constexpr function;
  • (5.5)
    an invocation of a virtual function for an object unless
    • (5.5.1)
      the object is usable in constant expressions or
    • (5.5.2)
      its lifetime began within the evaluation of E;
  • (5.6)
    an expression that would exceed the implementation-defined limits (see [implimits]);
  • (5.7)
    an operation that would have undefined behavior as specified in [intro] through [cpp]84;
  • (5.8)
    an lvalue-to-rvalue conversion unless it is applied to
    • (5.8.1)
      a non-volatile glvalue that refers to an object that is usable in constant expressions, or
    • (5.8.2)
      a non-volatile glvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;
  • (5.9)
    an lvalue-to-rvalue conversion that is applied to a glvalue that refers to a non-active member of a union or a subobject thereof;
  • (5.10)
    an lvalue-to-rvalue conversion that is applied to an object with an indeterminate value;
  • (5.11)
    an invocation of an implicitly-defined copy/move constructor or copy/move assignment operator for a union whose active member (if any) is mutable, unless the lifetime of the union object began within the evaluation of E;
  • (5.12)
    an id-expression that refers to a variable or data member of reference type unless the reference has a preceding initialization and either
    • (5.12.1)
      it is usable in constant expressions or
    • (5.12.2)
      its lifetime began within the evaluation of E;
  • (5.13)
    in a lambda-expression, a reference to this or to a variable with automatic storage duration defined outside that lambda-expression, where the reference would be an odr-use;
    [Example 1: void g() { const int n = 0; [=] { constexpr int i = n; // OK, n is not odr-used here constexpr int j = *&n; // error: &n would be an odr-use of n }; } — end example]
    [Note 3:
    If the odr-use occurs in an invocation of a function call operator of a closure type, it no longer refers to this or to an enclosing automatic variable due to the transformation ([expr.prim.lambda.capture]) of the id-expression into an access of the corresponding data member.
    [Example 2: auto monad = [](auto v) { return [=] { return v; }; }; auto bind = [](auto m) { return [=](auto fvm) { return fvm(m()); }; }; // OK to capture objects with automatic storage duration created during constant expression evaluation. static_assert(bind(monad(2))(monad)() == monad(2)()); — end example]
    — end note]
  • (5.14)
    a conversion from type cv void* to a pointer-to-object type;
  • (5.15)
    a reinterpret_­cast ([expr.reinterpret.cast]);
  • (5.16)
    a modification of an object ([expr.ass], [expr.post.incr], [expr.pre.incr]) unless it is applied to a non-volatile lvalue of literal type that refers to a non-volatile object whose lifetime began within the evaluation of E;
  • (5.17)
    a new-expression ([expr.new]), unless the selected allocation function is a replaceable global allocation function ([new.delete.single], [new.delete.array]) and the allocated storage is deallocated within the evaluation of E;
  • (5.18)
    a delete-expression ([expr.delete]), unless it deallocates a region of storage allocated within the evaluation of E;
  • (5.19)
    a call to an instance of std​::​allocator<T>​::​allocate ([allocator.members]), unless the allocated storage is deallocated within the evaluation of E;
  • (5.20)
    a call to an instance of std​::​allocator<T>​::​deallocate ([allocator.members]), unless it deallocates a region of storage allocated within the evaluation of E;
  • (5.21)
    an await-expression ([expr.await]);
  • (5.22)
    a yield-expression ([expr.yield]);
  • (5.23)
    a three-way comparison ([expr.spaceship]), relational ([expr.rel]), or equality ([expr.eq]) operator where the result is unspecified;
  • (5.24)
    a throw-expression ([expr.throw]);
  • (5.25)
    a dynamic_­cast ([expr.dynamic.cast]) or typeid ([expr.typeid]) expression that would throw an exception;
  • (5.26)
    an asm-declaration ([dcl.asm]); or
  • (5.27)
    an invocation of the va_­arg macro ([cstdarg.syn]).
If E satisfies the constraints of a core constant expression, but evaluation of E would evaluate an operation that has undefined behavior as specified in [library] through [thread], or an invocation of the va_­start macro ([cstdarg.syn]), it is unspecified whether E is a core constant expression.
[Example 3: int x; // not constant struct A { constexpr A(bool b) : m(b?42:x) { } int m; }; constexpr int v = A(true).m; // OK: constructor call initializes m with the value 42 constexpr int w = A(false).m; // error: initializer for m is x, which is non-constant constexpr int f1(int k) { constexpr int x = k; // error: x is not initialized by a constant expression // because lifetime of k began outside the initializer of x return x; } constexpr int f2(int k) { int x = k; // OK: not required to be a constant expression // because x is not constexpr return x; } constexpr int incr(int &n) { return ++n; } constexpr int g(int k) { constexpr int x = incr(k); // error: incr(k) is not a core constant expression // because lifetime of k began outside the expression incr(k) return x; } constexpr int h(int k) { int x = incr(k); // OK: incr(k) is not required to be a core constant expression return x; } constexpr int y = h(1); // OK: initializes y with the value 2 // h(1) is a core constant expression because // the lifetime of k begins inside h(1) — end example]
6
#
For the purposes of determining whether an expression E is a core constant expression, the evaluation of a call to a member function of std​::​allocator<T> as defined in [allocator.members], where T is a literal type, does not disqualify E from being a core constant expression, even if the actual evaluation of such a call would otherwise fail the requirements for a core constant expression.
Similarly, the evaluation of a call to std​::​destroy_­at, std​::​ranges​::​destroy_­at, std​::​construct_­at, or std​::​ranges​::​construct_­at does not disqualify E from being a core constant expression unless:
  • (6.1)
    for a call to std​::​construct_­at or std​::​ranges​::​construct_­at, the first argument, of type T*, does not point to storage allocated with std​::​allocator<T> or to an object whose lifetime began within the evaluation of E, or the evaluation of the underlying constructor call disqualifies E from being a core constant expression, or
  • (6.2)
    for a call to std​::​destroy_­at or std​::​ranges​::​destroy_­at, the first argument, of type T*, does not point to storage allocated with std​::​allocator<T> or to an object whose lifetime began within the evaluation of E, or the evaluation of the underlying destructor call disqualifies E from being a core constant expression.
7
#
An object a is said to have constant destruction if:
  • (7.1)
    it is not of class type nor (possibly multi-dimensional) array thereof, or
  • (7.2)
    it is of class type or (possibly multi-dimensional) array thereof, that class type has a constexpr destructor, and for a hypothetical expression E whose only effect is to destroy a, E would be a core constant expression if the lifetime of a and its non-mutable subobjects (but not its mutable subobjects) were considered to start within E.
8
#
An integral constant expression is an expression of integral or unscoped enumeration type, implicitly converted to a prvalue, where the converted expression is a core constant expression.
[Note 4:
Such expressions can be used as bit-field lengths ([class.bit]), as enumerator initializers if the underlying type is not fixed ([dcl.enum]), and as alignments.
— end note]
9
#
If an expression of literal class type is used in a context where an integral constant expression is required, then that expression is contextually implicitly converted ([conv]) to an integral or unscoped enumeration type and the selected conversion function shall be constexpr.
[Example 4: struct A { constexpr A(int i) : val(i) { } constexpr operator int() const { return val; } constexpr operator long() const { return 42; } private: int val; }; constexpr A a = alignof(int); alignas(a) int n; // error: ambiguous conversion struct B { int n : a; }; // error: ambiguous conversion — end example]
10
#
A converted constant expression of type T is an expression, implicitly converted to type T, where the converted expression is a constant expression and the implicit conversion sequence contains only and where the reference binding (if any) binds directly.
[Note 5:
Such expressions can be used in new expressions ([expr.new]), as case expressions ([stmt.switch]), as enumerator initializers if the underlying type is fixed, as array bounds, and as non-type template arguments.
— end note]
A contextually converted constant expression of type bool is an expression, contextually converted to bool, where the converted expression is a constant expression and the conversion sequence contains only the conversions above.
11
#
A constant expression is either a glvalue core constant expression that refers to an entity that is a permitted result of a constant expression (as defined below), or a prvalue core constant expression whose value satisfies the following constraints:
  • (11.1)
    if the value is an object of class type, each non-static data member of reference type refers to an entity that is a permitted result of a constant expression,
  • (11.2)
    if the value is of pointer type, it contains the address of an object with static storage duration, the address past the end of such an object ([expr.add]), the address of a non-immediate function, or a null pointer value,
  • (11.3)
    if the value is of pointer-to-member-function type, it does not designate an immediate function, and
  • (11.4)
    if the value is an object of class or array type, each subobject satisfies these constraints for the value.
An entity is a permitted result of a constant expression if it is an object with static storage duration that either is not a temporary object or is a temporary object whose value satisfies the above constraints, or if it is a non-immediate function.
[Example 5: consteval int f() { return 42; } consteval auto g() { return f; } consteval int h(int (*p)() = g()) { return p(); } constexpr int r = h(); // OK constexpr auto e = g(); // error: a pointer to an immediate function is // not a permitted result of a constant expression — end example]
12
#
Recommended practice: Implementations should provide consistent results of floating-point evaluations, irrespective of whether the evaluation is performed during translation or during program execution.
[Note 6:
Since this document imposes no restrictions on the accuracy of floating-point operations, it is unspecified whether the evaluation of a floating-point expression during translation yields the same result as the evaluation of the same expression (or the same operations on the same values) during program execution.
[Example 6: bool f() { char array[1 + int(1 + 0.2 - 0.1 - 0.1)]; // Must be evaluated during translation int size = 1 + int(1 + 0.2 - 0.1 - 0.1); // May be evaluated at runtime return sizeof(array) == size; }
It is unspecified whether the value of f() will be true or false.
— end example]
— end note]
13
#
An expression or conversion is in an immediate function context if it is potentially evaluated and its innermost non-block scope is a function parameter scope of an immediate function.
An expression or conversion is an immediate invocation if it is a potentially-evaluated explicit or implicit invocation of an immediate function and is not in an immediate function context.
An immediate invocation shall be a constant expression.
14
#
An expression or conversion is manifestly constant-evaluated if it is:
  • (14.1)
    a constant-expression, or
  • (14.2)
    the condition of a constexpr if statement ([stmt.if]), or
  • (14.3)
    an immediate invocation, or
  • (14.4)
    the result of substitution into an atomic constraint expression to determine whether it is satisfied ([temp.constr.atomic]), or
  • (14.5)
    the initializer of a variable that is usable in constant expressions or has constant initialization ([basic.start.static]).85
    [Example 7: template<bool> struct X {}; X<std::is_constant_evaluated()> x; // type X<true> int y; const int a = std::is_constant_evaluated() ? y : 1; // dynamic initialization to 1 double z[a]; // error: a is not usable // in constant expressions const int b = std::is_constant_evaluated() ? 2 : y; // static initialization to 2 int c = y + (std::is_constant_evaluated() ? 2 : y); // dynamic initialization to y+y constexpr int f() { const int n = std::is_constant_evaluated() ? 13 : 17; // n is 13 int m = std::is_constant_evaluated() ? 13 : 17; // m might be 13 or 17 (see below) char arr[n] = {}; // char[13] return m + sizeof(arr); } int p = f(); // m is 13; initialized to 26 int q = p + f(); // m is 17 for this call; initialized to 56 — end example]
[Note 7:
A manifestly constant-evaluated expression is evaluated even in an unevaluated operand.
— end note]
15
#
An expression or conversion is potentially constant evaluated if it is:
A function or variable is needed for constant evaluation if it is:
  • (15.6)
    a constexpr function that is named by an expression that is potentially constant evaluated, or
  • (15.7)
    a variable whose name appears as a potentially constant evaluated expression that is either a constexpr variable or is of non-volatile const-qualified integral type or of reference type.
83)
Overload resolution ([over.match]) is applied as usual.
 ⮥
84)
This includes, for example, signed integer overflow ([expr.prop]), certain pointer arithmetic ([expr.add]), division by zero ([expr.mul]), or certain shift operations ([expr.shift]).
 ⮥
85)
Testing this condition might involve a trial evaluation of its initializer as described above.
 ⮥
86)
Constant evaluation might be necessary to determine whether a narrowing conversion is performed ([dcl.init.list]).
 ⮥
87)
Constant evaluation might be necessary to determine whether such an expression is value-dependent ([temp.dep.constexpr]).
 ⮥